Probably most people here know this, but it's worth mentioning again that a GPS is only as good as the maps/waypoints that you have.
To make your own position (as shown by the GPS) useful, you need to ALSO have the position of other things - your destination, landmarks, junctions, or whatever.
Garmin's World Map is handy, but its accuracy ranges from perfect (in a few places) to hopeless (in most places). Consequently, it is very dangerous to rely on getting to a point marked on that map - the water or village or whatever may not be there at all.
Likewise, be a bit careful about waypoints provided by other travellers, unless they are corroborated. A small typo when they listed the waypoints could mean disaster if you are relying on the waypoint in the desert.
This doesn't apply when the GPS is really just there for convenience, but in remote/dangerous areas, never rely entirely upon a GPS for your navigation.
Wrong place to post this, I guess, but what harm can it do?