Do you need a GPS?
It's generally wise to be prepared when heading into unfamiliar territory, and a GPS can provide protection if conditions turn out to be less than ideal. Issues like bad weather reducing visibility, washed out trails, poorly marked trails (worse if combined with poor visibility). Fog, rain, or snow can all be problematic and make a trail finding task that would be easy in good conditions, turn into a situation where one can easily get completely lost. We have all more than once taken a "short cut" after descending a climb, following what was initially a pretty good trail, only to have the trail peter out and end up in an epic bushwack back to the car in the dark. A GPS can save you on early-season backpacking trips where snow might hide an otherwise obvious trail.
All this said, learn to navigate by map and a compass before you get a GPS. A map doesn't run on batteries and can't be broken (we like to put printed maps in 8" x 11" waterproof aLOCKSAK bags).
What are GPS useful for?
A GPS can provide valuable location-based information that you can't get with your own eyes. For example, marine GPS devices with charts show water depth, potential underwater hazards, and – most importantly – display your position in poor visibility conditions. Lobster fishermen in Maine use GPS to plot the locations of their traps so that they can fish even in dense fog, heavy rain, or whiteout snow conditions. Hunters use GPS to mark the location of their kill, tree stand, or trailhead. For outdoor recreation purposes, GPS devices are most useful for displaying your position (which presumably can't be found by other methods) relative to your destination. Luc Mehl has a good tutorial on GPS route planning for low visibility conditions here.
Smartphone GPS applications
Most of us carry a phone in the backcountry. So why carry something else?
Answer: dedicated handheld GPS have better satellite reception, more powerful navigation features, and better battery life than smartphones GPS applications; they are ideal for extensive off-trail navigation in low visibility conditions. However, if you carry a phone and primarily walk on trails or travel on established routes (like rivers), a smartphone app is likely all you need. We recommend the Trimble Outdoors app for its navigation features (requires additional purchase of area-specific maps) and the Topo Maps app for the best value (iPhone/iPod only, download unlimited free maps!! before you leave). Based on our testers' extensive experience in the backcountry we've found that a GPS is most commonly used to identify or confirm our position briefly before we resume navigating with maps. For such applications, an app like Topo Maps works well and only costs $8. For most backpackers we suggest starting out with a GPS app and upgrading to a handheld GPS if it proves insufficient for your needs.
What to Consider When Buying a handheld GPS
The primary question to consider when buying a GPS device is whether you need a touchscreen. Touchscreens are advantageous in that they provide faster operation than traditional screens. However, they're generally harder to see in bright sunlight, behave sluggishly in colder temperatures, many require bare fingers to operate, and they suck up more battery than a normal screen. (The exception is the Garmin Oregon 600. Most of the touchscreen GPS units we tested have displays that are years behind those of most smartphones.
Do you need a GPS capable of car navigation?
If yes, we suggest using a smartphone GPS app, like Google Maps, because it provides much better directions and traffic information than any handhled unit tested here. Many GPS manufacturers also make dedicated auto GPS units.
Electronic compasses are usually the cut-off point where manufacturers separate the basic units from the more powerful ones. An electronic compass is useful for navigating to an unknown point. Imagine you're in a gigantic white room. You can't see the walls, but know that the floor is level and that you must find an invisible location in the room, for which you have the coordinates in your GPS. With an electronic compass you'd be able to see the point on the GPS display, turn the device (in map view with track up) until the point lies ahead, then simply walk to it. Without an electronic compass you'd have to walk one direction far enough for the GPS to accurately identify the direction (usually 100ft) and turn again, and perhaps again, in the direction of the point. Electronic compasses make navigating faster and easier. They're nice, but not necessary.