Choose one fully-featured GPS app that includes maps of the whole world:
These apps include the fundamental feature set that make them useful tools for hiking anywhere in the world:
* Store OpenCycleMaps and Satellite Images for offline use.
* Ideally provides National Agency maps for the areas you most frequently hike (USGS, NRCan, LINZ, OS, IGN, etc)
* Import, Create, and Export tracks and waypoints (via gpx and/or kml)
  Gaia GPS Does all of the important stuff with little extraneous clutter. In addition to the most extensive selection of map sources, it has a cleaner user interface thanmany of the competing full-featured apps (such as MotionX and GPS Kit). After extensive use in the field, we have come to the conclusion that Gaia is currently the best option for a fully-featured app.
In addition to the basic features (display variety of maps, import/create/export tracks and waypoints), Gaia includes some very useful features that might not be obvious.
1) Downloading all maps for a track (new in Oct2011) is a FANTASTIC addition for long-distance walkers, but it's not easy to find -- from the list of Tracks, select a track and then scroll down to the bottom of the Track detail page
Download Maps for Track.
2) In addition to the map sources that are available when you first install the app, you can add many more sources at no charge. From the map page, tap the Layers icon, scroll down and tap "Add more Layers"
2) There are four sources of downloadable satellite imagery. Some sources are better than others, depending on the location.
3) Downloadable New Zealand LINZ maps (new in Dec2011).
4) From locally stored files in Dropbox, you can import gpx files even when offline. To import kml files from dropbox, you must be online.
5) You can create routes from inside the app. From the Map screen, tap the Waypoint Flag icon and then tap Create Route.
6) You can maximize battery conservation by configuring the app to find your GPS location only on demand (instead of every time you open the app, which is what most map apps do). Settings->Power Saving->No GPS Until Activated.
7) Gaia Cloud, where you can backup your maps and tracks, sync maps and tracks across iOS devices, and share data with friends.
Map Sources:
National Agency Maps: USGS, NRCan, LINZ
Satellite Imagery: USGS Aerial, ESRI World Imagery, MapQuest Imagery, Google World Imagery.
OpenStreetMap: Topo, Road, Shade, and a few others
Additional map sources: see http://www.gaiagps.com/map_sources?app_type=gaiagps - the list includes NPS Visitor overview maps, USFS maps, and historic USGS maps from the 1930s
Additional charges: There is an addon called "Gaia Green" which offers some fancy extras, however none of the core functionality is hidden behind this extra fee.
Import GPX/KML? Yes, via email or Dropbox or iTunes. Export gpx/kml via eMail.
Bugs: (No battery draining bugs as of version 5.2, if you have a prior version be sure to update.)  
More great apps. These apps fill a niche, but have limited  functionality and/or geographic coverage.
  Maplets Easily download a wide variety of maps, such as NPS and USFS maps, local/state/county park maps, transit system maps, town maps, airport maps, cycleways, etc. Many of these are geographically calibrated, so that your location appears on the map, others (such as transit maps) just provide an offline copy of the paper map. For hikers, this is useful when park maps include trail and visitor information that is not on the topo maps. The app does not have any features other than showing your current position on the map, so it is not a replacement for a fully-featured map/gps app. It is extremely easy to use, with a nice simple interface to find and download maps. I love this app.
The inventory of park maps for the US is very thorough, less so for the rest of the world. With each passing week the inventory of available maps grows. If you know of a map that should be included submit it and the developer will work to add it; they added all the maps from WalkingCoastalSydney.com.au within a few days of my request! Very cool app, I think everybody will love it, and the more people who use it the bigger the inventory of maps will become.
Map Sources: Huge variety of maps. See mobilemaplets.com/places
Additional charges: None. All maps included in original purchase price.
Bugs: (No battery draining bugs!)
  Google Maps The Google Maps app has a hidden feature that makes it useful for travel.  Simply enter OK Maps into the search box when viewing the area you want for later, and that region, in all of its zoom levels, is saved for offline use.  This does not make it a good tool for backpacking (no topographic maps, no track/waypoint importing, etc) but it is a terrific feature that enables  use of the app while offline.
  Topo Maps USGS and NRCan maps of US and Canada. Wow, this is a fantastic app! Everybody hiking in US or Canada should buy this one. Map downloads intuitive and easy. Searching for geographic locations is best in class. Distance/bearing tool is best in class. UI design is great - easy to learn, easy to use, very polished. Waypoint mgmt is excellent, but the app does not handle imported tracks well, and does not display or create tracks. Importing waypoints is limited to downloading from URL (not as easy as via iTunes or email or dropbox).
This app only offers
one map source, does not display or record tracks, and does not have a simple way to import waypoints. However, it is a great app for the limited set of functionality it does provide.
Map Sources: USGS 7.5 and comparable NRCan maps of Canadian.
Additional charges: None. All maps included in original purchase price.
Import GPX? Imported Waypoints are handled nicely, but limited to downloading via URL. Imported tracks are not handled well. Importing files via iTunes or DropBox are not supported, and the process to import is not as intuitive as it should be.
Bugs: (No battery draining bugs!)
  ViewRanger ViewRanger is a robust full-featured mapping application. In some locations it is the best product available.
For the U.K. this is the best app for using Ordnance Survey Maps.
For the
U.S. and Canada and NZ Gaia is a better choice because it offers a much wider selection of map sources, including satellite imagery, and downloading maps and switching between map sources in Gaia is much more straight-forward.
For
France, iPhiGeNie offers a complete selection of IGN maps with a clean UI and better price (annual license for every map IGN publishes is modest).
For
OSM maps of the world, Gaia is superior (proper download and storage of map tiles for offline use, whereas ViewRanger caches 1 gig of recently viewed map tiles for up to 90 days, and switching between OSM and National Agency maps in ViewRanger is not straight-forward as it is in Gaia).
For remaining countries where ViewRanger sells government issued topo maps, this may be the best solution available for viewing those maps (I haven't searched for other apps that display these map sources). This includes:
Belgium, Finland, Germany, Ireland, New Zealand, Netherlands, Norway, Slovenia, Sweden, Switzerland, Czech Republic.
The pricing structure is based on number of tiles downloaded; I didn't study enough to know how that translates in real life.
Map Sources: Premium maps (pay as you go) from national mapping agencies for much of Western Europe, New Zealand and the US.
OSM Maps are available for free, but are cached rather than properly downloaded and saved.
Additional charges: ViewRanger charges a fraction of a "credit" per tile, making extremely difficult to estimate how much it actually cost to use the product. My downloaded maps of Turkey, for example, used over ten thousand tiles, and I have no idea how I would estimate in advance how much it would cost to use ViewRanger's Premium maps for a 500 mile hike in Europe.
Import&Export GPX? Yes.
  UK Map High resolution OS maps of the UK. OS has not released LandRanger or Explorer map data as part of their free scheme, so the maps in this app are not as good as the maps available from ViewRanger. However, the maps are very good for the price (all map content is free) and will suffice for many users. Although made by the same developer as Topo Maps, this app does not have the same feature set. UK Map has a well implemented 3D mode (not in Topo Maps), but is missing distance/bearing ring and has no waypoint or track features. Excellent UI, easy to learn and easy to use, but not full featured like ViewRanger (for example you can't import gpx/kml files).
Map Sources UK Map: O.S. maps, 1:10K StreetView and 1:50K (similar to but not as thorough as LandRanger series, with OpenStreetMap trails superimposed on maps.
Additional charges: None. All maps included in original purchase price.
Import GPX? No.
Bugs: I have not tested yet
  Offline Topo Maps  This is made by Gaia, and is essentially Gaia GPS without some features. If you want to download and store a great selection of map sources, and don't need to import gpx files or create tracks, this is a terrific choice. You can create and export waypoints, but not tracks. Very straight forward easy UI. It's a good option for price-sensitive folks who don't need to import gpx or create tracks.
Map Sources: same as Gaia GPS
Additional charges: None. All maps included in original purchase price.
Import GPX: no. Export gpx/kml via email.
Bugs: (No battery draining bugs!)  
  iPhiGeNie Only covers France, French territories, and Spain. Very nice clean user interface. I wish I'd had this for our 2010 HRP hike and 2012 hike in southern Spain. Note that I have not used this in the field, but I have been very impressed evaluating it at my desk. I wish it covered areas other than France and Spain. Annual fee for use of map data is reasonably priced.
Map Sources: All IGN (France and Spain) Maps and Satellite images of France and French Territories are available! See www.geoportail.fr
Additional charges: 5 one-time charge for upgrade that includes import/export gpx. First week of map use is free (trial, or short trip) and then it cost 14 per year fee to use all maps.
Import GPX: yes
Bugs: (I have not yet tested for battery draining bugs)
  National Park Maps HD (from National Geographic) Includes only 20 US National Parks. No import of gpx and no Tracking feature. But for these parks, it has excellent maps (Trails Illustrated, USGS based with 3D shading and trail distances) and good additional information. This would be a nice supplemental resource when visiting the parks that are included. Includes ability to add waypoints, but no Tracking feature, and no ability to import gpx files.
Map Sources: High quality Trails Illustrated Maps of 20 US National Parks
Additional charges: None. All maps included in original purchase price.
Import GPX: no
Bugs: (No battery draining bugs!) 
  Marine Charts (by EarthNC) High resoultion USAA Marine Charts, weather info, tide charts, and other info geared to boaters. Made by Gaia, so shares the same clean user interface.
Bugs:  (I have not yet tested for battery draining bugs, but likely same as Gaia)
Possibly best available app for certain locations or situations:
  NeoTreks GPS (aka AccuTerra or AccuTerra HD by NeoTreks) Only for the US.  Only one proprietary map source, from AccuTerra.  Nice shading to give 3D representation of topography. 
Some users report liking the AccuTerra maps more than USGS maps, and so I include it in the "Possible for some areas" list of apps.  Personally, I downloaded one section to evaluate, and I found it inadequate.  For example, Ericson Crags are labelled "Mt Brewer" (which is 3.7 miles away), and Mt Brewer and Mt Ericson have no labels at all.  These are major peaks in SEKI NP! Maybe I randomly chose to look at the only place that is badly inaccurate, I don't know.  In my opinion, better to use a combo of USGS (for completeness and accuracy) and OSMTopo (for more recent cultural additions). and Satellite imagery - AccuTerra has none of these.
  Bit Map Bit Map is an offline map viewer for your own topographic or specialised maps and fills a niche that isn't filled by any other app in this list.  If you have maps (many digital formats are supported, or scan your paper maps) you can use them in this app.  Supports importing and explorting kml files, recording and displaying waypoints and tracks.  It looks like a good solution for viewing maps that are not available via other apps.  Note that this is not as easy to use as other apps, since you must first do several steps to massage your digital files into the format Bit Map requires, however this effort is the natural price you pay for using your own map data, and the reward is that you can use maps not available via any other app.
Note: I have not tried this yet, as I don't currently have a need to use my own map content and I am only testing/evaluating apps that meet an immediate need.
  Maps Viewer for large maps in pdf format. It doesn't provide any map features - no scale, doesn't show your location on the map, nothing. But it allows you to easily load and seamlessly pan and zoom extremely large pdf files. I tested it by comparing it to GoodReader, the app I normally use to view pdf files.
Using a relatively small 372 KB pdf file: GoodReader retains full resolution, so the map is crisp and clear, whereas Maps resolution is slightly degraded. Maps, however, has no delay when panning the map as GoodReader does.
Using an 8.1 meg pdf file (Los Padres National Forest northern section) crashes in GoodReader, but works fine in Maps! USFS provides pdf format maps for most or all USFS units, and Maps makes it possible to view those files.
If a map (such as USFS maps) is already available via Maplets, then use Maplets instead (since Maplets shows your location on map); if it ought to be in Maplets but is not yet available there, then request that it be added! If it is proprietary map (not publishable on Maplets) and you don't want to invest the time to use Bit Map, then this app provides an easy way to view it on the iPhone.
Note that the only two PDF viewer apps I tested are GoodReader and Maps. The maker of Maps, Smalltalk Consulting, also makes an app called Large Viewer which handles huge files in
dozens of formats (jpeg, tiff, etc) and would be a more generalized solution (I now wish I'd bought Large Viewer instead of Maps).
  The complete National Cycle Network (UK) This is a map of the whole UK, not just the bike routes themselves. Note that I downloaded and briefly explored this app, but I didn't evaluate the feature set nor try to actually use it. 25,000 miles of bike routes are highlighted - including all 13,000 miles of National Cycle Network and a further 12,000 miles of regional and local routes and links. The detailed zoom level displays OS maps that include field boundaries and trails (which are not included in UK Map), which makes this app useful for hiking as well as cycling. Maps can be downloaded for offline use, although the download is limited to a rectalinear area with screen proportions - good for hiking in a region, but not a viable download method for long linear routes. If the download process is fixed to allow for a user-defined area, instead of a screen-shaped area, then this will be a good source for OS maps!
  MUD MAP - Outdoor Off Road GPS (Australia) Access to 1:25K topographic maps of Australia. Way too expensive for me to try it. $55 USD for the app. Plus $3 for a single NSW quad (7.5x15 minute). Holy cow, Australians are paying a fortune! Is there a better app for detailed Australian maps? Compare to Topo Maps ($8) or Gaia ($25), both of which include every quad in US and Canada. Or UK Maps ($12) which includes entire UK. Or to iPhiGeNie (20 per year) which includes every map IGN publishes.
  GPS Kit I used GPS Kit and Gaia about 50/50% on a Sept-Oct 2011 six week hike in Australia. Each had some things it did better than the other, so I used whichever one met my needs at the moment. At the time I recommended both apps.
However, more recent versions (6.2 and 6.3) of GPS Kit now have the battery drain problem (see Warning below), making it severely problematic for backpackers. Additionally, in the subsequent six months Gaia has made signficant improvements. Gaia is now a far superior choice for a fully-functionaly app. Numerous subtle design characteristics in GPS Kit make it harder to use while in the field, whereas Gaia is very usable. However, some people may prefer GPS Kit based on nuanced differences in feature set, for example, the ability to manage tracks is better in GPS Kit than in Gaia, although recent improvements in Gaia have narrowed the gap.
Gaia has USGS and OSM and Satellite images. GPS Kit has OSM and Satellite images, but no USGS maps.
Gaia has OSM POIs, GPS Kit has Squawk; I don't use either of those features but the difference could be important to others.
Gaia and GPS Kit are both better solutions for serious hikers than the less expensive MotionX.
Map Sources: OSM (Street and Cycle), Bing (many choices including Sat Images), Google (many choices)
Additional charges: None. All maps included in original purchase price.
Import GPX/KML? yes, via email or itunes
Warning: If app is left in foreground when iPhone is asleep, there will be a ~5% battery drain per hour. (this problem did not exist in version 6.1, but is present in 
  MotionX-GPS [not reviewed since summer 2011.] Full featured GPS application for OSM maps and Satellite images, but does not provide USGS maps or any other national agency maps. Some people might like this more than Gaia or GPS Kit based on nuanced differences in feature set or UI. Personally I find the other two apps more powerful, more intuitive and easier to use. You could try and decide for yourself.
Map Sources: OSM (aka MotionX): Road, Cycle; NOAA Marine (low resolution); Bing ( No access to USGS maps, which is a big downside for North Americans. (others sources available for online use, but not for downloading)
Additional charges: None. All maps included in original purchase price.
Import GPX? Yes, but maximum of 400 waypoints, and will not import Routes (must convert to Track first)
Warning: If app is left in foreground when iPhone is asleep, there will be a ~3% battery drain in the first ~60 minutes, at which time the app puts itself in background and battery drain stops. You must put the app in the background every time you put the device to sleep in order to conserve battery.
Not Yet Evaluated, may be useful for hiking:
  Avenza PDF Maps Looks promising, but I haven't tried it yet.
  Trimble Outdoors Navigator New app in Nov 2011.  The free version does not let you download maps for offline use.  TrimbleOutdoors.com says that feature is available in the Pro version.
  MapsWithMe An offline OpenStreetMap app.  Includes map plus all the POIs, downloaded for offline use.  Compare this to OpenMaps Pro
  mophie outdoor new in Oct 2011
  Topographic Points Canada Downloadable NRCan topo maps.
  TopoPoint Canada Downloadable NRCan topo maps.
  AllTrails Integrates with AllTrails.com, a source of trail info.  I didn't study this yet and have no comments
  Trails Integrates with Trails.com, a source of trail info.  I didn't study this yet and have no comments
  Memory-Map I have not studied this yet
  Fugawi iMap: USA&Canada Topo Maps I have not studied this yet
NO:  These appeared to have potential for hikers.  I ruled them out for reasons listed.
  B.iCycle Close, but map storage is limited to cached tiles.  Focus is Cycle trip recording.  Good reviews from cyclists. 
  Backpacker GPS Trails App cost $10 and there is no free reduced-funtion version to try.  I can't find a user manual.  I don't know what types of maps are provided, and it appears that maps are cached and not properly downloaded.  I'm not going to spend the money to experiment, as the user reviews are not very encouraging and I'm put off by the lack of information.
  CycleStreets for Iphone bike routing, focus on city biking, UK only.  Not using Retina Display resolution.
  Galileo Offline Maps  The only feature is the ability to see your current position on a custom map (maps made on desktop machine using Mobile Atlas Creator).  Prior to June 2011 MobAC allowed you to create satellite image maps, but this is no longer true.  Without ability to use Satellite images, this app is more complicated to use and the app itself is woefully lacking in functionality.
  GeoGuide 3D WorldWide Maps Download OpenStreet and OpenCycle maps for offline use.  But you can only specify one of three levels of detail, unlike Gaia or MotionX where you have better control over the maximum zoom level.  Import gpx files via email, DropBox, EveryTrail, which is great.  I found the UI unintuitive, seemed like a constant struggle to get the right maps and tracks to display.  You can turn off 3D, but even in 2D there is always shadowing superimposed on the map, which makes i it harder to read.  Two finger zooming changes the orientation of the map, yuck!  It looks like the intention is for the user to use the + and - buttons to change zoom levels, but that's not nearly as easy as two fingers.   It's "cool" to be in 3D and rotate the map around, but for real navigation use it needs to stay in one place (user option of North up or oriented to the ground, like Gaia and the other good apps offer).  This is not a good tool for serious navigation, but it might be a fun app to have for 3D entertainment purposes.
  GPS Tuner Offers nothing that Gaia, GPS Kit, and MotionX don't offer, and lacks gpx file import, and has fewer map sources.
Map Sources:  OSM Sreet and OSM Cycle (others sources available for online use, but not for downloading)
  iHike GPS UI is clumsy.  Changing zoom level has a lag that makes it nearly unusable.  USFS maps do not add substantive info compared to USGS maps, and this app has no advantages over Gaia and Topo Maps for USGS maps.
  iTopoMaps There were so many iTunes reviews that said the app is unstable and unreliable that I did not even evaluate this one.  Also, the web-site forum had no entries, and the FAQ was very anemic. 
  MapTap no documentatation, AWOL developer, bad reviews
  Hike&Bike (by Navionics) Product description does not mention downloading maps for offline use, import or export of gpx files, available map sources.  It says it's designed for hikers, but it doesn't address any of our needs.  Their website has no user manual or product description.  This looks like an anemic after-thought from a company that specializes in marine charts and apps for boaters.
  OpenMaps Pro The inability to define the download area as a long narrow shape is so onerous as to be a show stopper for hiking long routes.
No GPX download.
Unable to delete saved maps one at a time.
  RouteBuddy Atlas OSM maps are cached, not stored.  USGS maps are $30 per state (Topo Maps app and Gaia app offer those for the whole country at no additional charge).  OS Maps are 20 pounds per region (avalable from UK Map app at no additional charge)
  Trail Maps by National Geographic Coverage limited to Continental U.S. First released Aug2, 2011. This app lets you download both USGS quads and aerial images for offline use. It's easy to use, and from what I have seen it appears stable.
There are, however, two better apps for USGS quads: Topo Maps and Gaia. Compared to them, Trail Maps has less functionality. If you just want USGS maps and don't need to record tracks, then Topo Maps is hands down the best option. If you want USGS and Satellite, then Gaia is a better choice than Trail Maps because you get more functionality, more map sources, and it doesn't have the battery draining bug.
Map Sources: USGS 1:24K and 1:100K for lower 48 states. And Bing Aerial Images!
Additional charges: None. All maps included in original purchase price.
Import GPX: no. And no GPX export either.
Warning: If the Location Arrow is engaged/green then the battery will drain ~5% per hour if app is left in foreground, but if Location Arrow is not engaged/green then there is no drain.  (There might even be battery drain if the app is put in the background with the Location Arrow engaged, I haven't finished testing.).
  Trails - GPS Tracker No advantage over MotionX or Gaia.  UI is confusing because it assumes you are basing the hike on an existing gpx file, so the map is displayed by choosing a gpx route to dispay.  I can't figure out how to just look at a map (other than my current location) without first importing a gpx.
Downloaded maps can only be completely cleared, no way to manage your library of maps.
Not for Hiking:  Special Interest and/or Limited Focus Apps Not Suitable for Hiking
  700 City Mapsaka Mobile Streetmaps offline CITY MAPS
  ArcGIS for iPhone OSM editing tool
  aSmartHUD NAVI MAP+ navigation, car and pedestrian
  Bike Hub Cycle Journey Planner bike routing, focus on city biking, UK focus
  Bluemapia all about boating
  BTC Mapper OSM editing tool
  CityMaps 2Go offline CITY MAPS
  DirectU maps (one app per continent) Simply a map of one continent.  Like having Rand McNally World map in your pocket with no need for WIFI or ATT/Verizon access.  The size of each continent's app is huge, but you don't have to decide in advance which region you want to save for offline use.  If you have room on your iPhone and you want a world street map, take a look at it.  I've been using it and have been happy (and hey, it's free).
  EasyMaps offline CITY MAPS
  ForeverMap ForeverMap is an offline-navigation app, which Includes Wikipedia articles and display the OSM-Map.  You download one state at a time, and with the download you get the detailed street map plus POIs and short Wiki entries.  The routing feature is bad (takes shortest rather than fastest), but it's nice to have a state map with all the POIs available for offline use. I've been using it and have been happy with it.
  Global Navigator Pro This could be used instead of Galileo to view maps created with Mobile Atlas Creator.  It has more features than Galileo, but the interface is confusing, it does not import gpx tracks, and it doesn't display a scale.  Even though Galileo has no features at all, I found it easier to use than this one.
  GPSei Mobile Application social networking focus - Facebook and Twitter.
  GPSies just records your path. (no map displayed)
  iGO primo (North America) Next generation from DirectU.  In addition to the basic offline street map of a whole continent (the DirectU map), this app has routing and navigation features.  It might be a useful tool for road trips, when the google map built in app is not useful because you are offline.  Lots of features, might be useful if you spent the time to figure out how to use it.
  iLOE  OSM editing tool
  iLOS OSM Map and POI finder.  Search tool for OSM POIs
  Layar Reality Browser Argumented Reality browser mixes camera image with virtual reality
  Locations Tagger OSM editing tool.  no map displayed, just records a track
  Mapzen POI Collector OSM editing tool
  NavFree navigation
  Navit navigation
  OffMaps 2 City maps.   Offline maps and POIs for selected destinations around the world. Combines OSM map with Wikipedia Articles.
  Omaps Bad reviews.  No documentation.  Apparently small limit on number of tiles in each map download, not even enough to a city at a time.  I didn't spend the $2 to try it.  includes Cloudmade, maybe topo but I can't tell without purchasing it first
  Open Car InfoTainmenT system navigation.  Focus on cars
  OSMTrack OSM Track Editing Tool for recording and uploading TRACKS (not just POIs)
  Path Tracker just records your path
  Roadee routing/navigation
  RoadMap car navigation
  Skobbler Navigation navigation
  The Map UK and Ireland Only
  Traveling Salesman Route Planning for commercial drivers.
  True Offline Maps  TRY THIS FOR TOWNS.  Offline OSM maps with navigation and POIs.
  UK TravelOptions viewing with tilting/spinning 3D buildings & transport data.
  VectorMaps GB UK Only
  VoxTrek Navigation GPS Navigation for blind users
  Wheelmap Wheelchair access points
  Wikimap Displays OSM POIs of alimni.net.  I didn't test it because it doesn't preload gpx files
Evaluation Criteria:
  What maps are available?  I care about topo maps and satellite images and specialized park maps.  Many apps provide OSM Topo (aka Cloudmade Cycle, OpenCycleMap, MotionX Terrain, etc), the free wiki style topo map of the world.  A few apps (iPhiGeNie, Topo Maps, Gaia, ViewRanger, Mud Map etc) also offer government issued maps (USGS, NRCan, IGN, OS, LINZ, etc) which tend to be more detailed but are limited in geographic coverage.  Some apps have proprietary map content (National Park Maps HD, Maplets). 
  How easy is it to store maps for offline use? I want to download maps via WIFI without being attached to a desktop computer, so I can get new maps while on the road. I want to be able to easily specify a region and set of zoom levels and have all the necessary map tiles downloaded. I need to specify long skinny regions, suited to long distance routes, not regions shaped like the iPhone screen (The complete National Cycle Network would be a good app excepting for this problem). I need to be able to see my Tracks when I specify my download region.  Ideally, I would be able to specify a Track and have the app figure out which tiles, I have seen this feature in Gaia GPS and EveryTrail.  "Caching" maps is not acceptable -- this is the process of saving map tiles that you have already viewed, which would require you to view every place at every zoom level prior to starting your trip, and is too difficult to confirm you have everything before starting a long hike.
  Import and Export gpx and/or kml files. I want to create my routes and waypoints using desktop tools (or use gpx/kml files acquired from friends or websites) and get those files into the app via a WIFI connection.  I want to email gpx/kml files from the app to my friends.  Nice to have:  the emailed gpx/kml file includes a link to view it on a map, so my friends don't need to know how to load them into an app.
  Tracks and Waypoints. I want to create waypoints.  Creating Tracks doesn't matter to me (too battery intensive for backpackers).  I want to control which Tracks and Waypoints appear on the map, control their colors, group them into folders for easier management, and sort the list by Distance-from-current-location (or possibly from map center), or Name.  I want to be able to choose a waypoint from my list and show it on the map (i.e. jump to that location).
  Target Waypoint. I want to choose a target waypoint, and easily monitor the distance and bearing to that target, either in the main map view or one tap away.
  Viewing the map.  Show a scale (but not the dumb way MotionX does it!).  Show a compass bearing.  Landscape/Portrait rotation.  Give me a way to measure distance and bearing (Trail Maps by National Geographic and Topo Maps are good).  Don't clutter the map screen with extraneous crap, either let me customize what shows up (OpenMaps has nice customization design) or auto-hide the controls (ViewRanger and iPhiGeNie), or let me manually hide the controls (Gaia and Topo Maps).  Panning and zooming in and out must be easy and fast enough that I don't perceive a delay.
  Ease of Use and Ease of Learning.   Give me good help content in the product (not via link to web site).  Respond promptly if I ask for help.
  Battery Drain. If I don't turn on Tracking or BuddyBeacon/Squawk, please pretty please don't drain my battery when the iPhone is asleep.  This is a horrendous problem for backpackers. As of July 2011, at least the following apps are guilty:  Trail Maps by National Geographic, MotionX, ViewRanger, EveryTrail.
By leaving the GPS engaged when the iPhone is asleep, the user is able to get an instant read on their location anytime they open the app.  The price is severe battery drain.  Either the app needs to disengage the GPS whenever the iPhone is asleep, or provide a "battery conservation mode" setting so that battery conscious users can operate in that mode.  It is not adequate to require the user to manually disengage the GPS with every use.
  Stability. For backpackers, this is a real tool, not a toy.  Cosmetic bugs are OK, but the important core functionality needs to be reliable.
  Pricing. I want transparent pricing.  Charge me whatever for the app.  If necessary charge me a clearly stated annual license fee to cover the cost of expensive map sources (for example, iPhiGeNie annual charge for IGN maps), or a one time charge for access to new map sources.  But I don't want to pay per tile or pay per minor region, such that it just continues to creep up on me, especially if the ongoing costs are hard to forecast.
  Things I DON'T care about. I don't use, and therefore didn't evaluate, the following things.  Social Networking (integration with twitter, facebook, squawk, buddy beacon).   In-app route planning tools (I use Google Earth, BikeRouteToaster, and GPSBabel).  Search for locations.  Creating Tracks and all of the trip statistics that go along with that (too battery intensive). Integrating photos.  OSM POIs.  I'm not implying that other people won't care about these, but they are not core backpacking functions, and I am only evaluating the things that actually matter to me :)