Very pleased with my TomTom GO920 performance especially in Europe, took us straight to our hotels. Plays music through radio and has clear instructions.
Battery life outside of the car is useless. Updating via computer is very easy.
I've hadn't used a TomTom before, but my Garmin works well. Had it for 3 years now without hassle. Maps are very professional and updated twice a year. Also the POI available from Garmap online are a great help with traffic camera's, speed limits, toll fee's etc...
I previously had a Garmin Nuvi 200 (?) that worked pretty well. Cheap, functional & easy to use. Never updated the maps in the time I had it though (only had it for about a year before some fsuckers jacked my car - it was in the cubby).
I would most definitely go for the TomTom, fewer click to get where you want to be and more regular updates on their maps, almost quarterly. Read somewhere that Garmin also buys their maps from TomTom, and seeing as TomTom own 51% of the global market share on navigation equipment making them the global market leader, I think the choice is obvious.
Depends on how much you're paying. I've found in the "cheaper" range [R2500ish] , Tom Tom gives you alot more bang for your buck in terms of what the software can do [particularly the ability to create pre-defined routes with multiple waypoints in advance] . On the high end [R5000+] i can't really comment.
With tom tom (tomtom XL) you get more info on screen, you are able to make changes to the map and share them. plan a route with way points, install additional POI's, customise startup and shutdown scrrens, it gives you ample warning before a turn (my friends garmin tells him to turn only on the turn).