Used Motion X GPS on an iPhone 4 through a four state trip. Kept it in my camelback and it always held a track. Got about 5 hours a day with it. If you already have an iPhone, you can’t beat the $2.99 price over a Garmin.
Loved it so much, I’m getting a bike mount. I just like it for rough estimates and to keep from getting lost. I’ve never checked how exact it is to position and distance compared to Garmin. I’ll bet even Garmin isn’t as good for speed and distance as a cat eye.
Tips: Import the gpx files available on singletracks, then "follow" the track on your ride. You’ll always know which branch to take. You can also download "offline" maps ahead of time if you don’t have a gpx trail map available for areas you won’t have signal on.
Also let’s you take photos of "waypoints" you can label if you don’t visit the trail often.