Everything invisible said and a few more. I like the "course track" feature when I am riding trails and old logging roads. Most of those aren't shown on GPS, even ones loaded with topo and local maps. I can usually find my way out, but on a few occassions, I have followed my course (via the GPS) in reverse.
The Garmin ones are the "Bee's Knees", but there are probably someother good ones as well. Checkout the Garmin Website or West Marine.
Good news- none of them are as expensive as the cheapest plane versions: but you can't do the ILS approach on it either.
Marl