Also, if you don't have a map, and don't know how to use it then you're pretty screwed if your GPS batteries go flat or you drop the thing.

The only time I found a GPS really useful in the mountains was after a friend fell 600ft in the Pyrennes.

I called up the rescue services on a mobile and when they asked, "Where are you?", I gave them the exact position with the GPS and a helicopter was there five minutes later.