The TomTom units are pretty decent, if you plug it into your computer (and sync it with TomTom home) it updates the satellite info and you can get a much quicker sat lock when you leave home. (I think that works for the next 7 days) also you can modify it a lot.
You can and add 3rd party apps (
http://www.webazar.org/tomtom/plugins.php?lang=uk for a list) change the menu's around to your preferance (3rd party menu editior) and change the start up and shut down screens, the day and night colors, the voice (I had Mr. T on mine for a bit) and more.
The lower models don't synthesize the name of the street (just says like turn left in 300 meters) and the POI (Points of Intrest) directory is a little lacking at times (At least in Canada) but you can add faviourites in a variety of ways, which kind of makes up for it.
If you miss your exit, sit tight and it'll recalculate another way for you to get there (Nice feature) or tell you to turn around.

I haven't tried the traffic add on, but that seems to be a nice feature but I'm pretty sure you pay monthly for it.
Anyways, TomTom and Garmin seem to be the big players, the Garmin units are supposed to be a little better, but for me, my TomTom has allowed me to drive aimlessly without feeling the least bit lost.
