GPS Overview: Recommend eTrex Legend
A lot of good advice here. So I'll only try to add some simple observations.
First, I give a good summary of the Garmin GPS line usefull for MTB
in this post on
this thread.
I use the GPS for many purposes.
Here's a list of some. Some are for play. Others are very useful, and possibly life saving, especially when doing solo rides in new areas. Navigating a new ride, and recording the ride in all sorts of ways (3D topo, profile, distance, climb data, ...) are the biggest values. I find new routes and am willing to explore new places knowing how to get back to a previously marked trail. I'm continuing to have fun with the GPS even after 3 years of use. I very much want to get even a better unit, with bigger color screen, more memory, better receiver, etc...
It's good to carry backup paper trail maps, especially because they can give you the trail name that matches the signs you often find. But I find that putting waypoints at intersections help me make sure I don't miss turns and head off into oblivion. Staying on a planned route is especially important if someone has to come look for me. I leave a map with my planned route with my wife when riding solo. Finding new trails, and even the trail head is nicer with the GPS. It's also fun for geocaching with friends and kids.
My eTrex Vista can go for about 8 hours on rechargeable NiMH batteries. I carry a spare pair of AA batteries anyway, for my flashlight and other emergency tools. The eTrex line does very well on the handlebars, and many of them can show maps. The eTrex does occasionally loose signal on the north side of deep canyons with trees. That's not very often really, and it usually comes back within a short distance of riding. The new SiRFIII receivers like on the small Edge and GPSMAP 60CSx would be good, but those units are from $300-$400, or more.
The small Edge can't take AA batteries, and doesn't display maps. It won't help you navigate, but more features for training. But it's good for tracking and recording ride data on the computer, especially with the better SiRFIII receiver. It's expensive though ($300)
The wrist mounted units, Forerunner and Foretrex, are moderately expensive ($200) and similar or less than the Edge in features, and with less receiver sensitivity.
My recommendation: For lower cost with good general function, look at something like the eTrex Legend for $120. It can display maps that are useful if you're lost (and even nicer if you buy the downloadable Topo map software for $80 more).