You're talking about a car-based GPS, not a handheld right?
I use the google maps program for blackberry (which has GPS built in). Love it, will never have a phone without it again. It's always on you, no matter who's car you're in or who you're traveling with.
My girlfriend has a tom-tom, and my blackberry came with navigation software with a moving map display etc and they're basically the same. I dislike the moving map displays because they seem to always show the wrong level of zoom... I like to know what's coming ahead, not all of a sudden get a "turn left ahead" message in heavy traffic when I have to cut across 4 lanes. Google maps lets me do that by giving more control over the zoom level, and also lets me quickly plot detours (with my brain, not a stupid computer that gets all screwed up if you deviate from its plan).
Google maps is the way to go; those car thingies are too dumbed down for me.