I recently bought the Nuvi600 after doing about 2 weeks worth of testing and research (too expensive to just go out and drop the cash without a bit of testing). I actually bought the TomTom 910 first, and tried it out on several work trips over several days. I wasn't impressed with the battery life and the POI database seems very outdated and inaccurate (I updated it immediatly after unpacking it). I returned that unit and went to the Nuvi660. It's significantly more expensive, but I feel it's worth it. The screen is slightly bigger and seems to be brighter and have better color. Battery life is insane...much better than the Tom. I also updated the software and had no problems with weird directions or missing POIs.

One edge that I will give the Tom is its 20GB internal hard drive. Obviously that gives you the ability to store lots of data (mp3s, etc.) in there. The only thing that concerns me about that is that hard drives eventually die. The Garmin has a 700MB internal flash memory and the ability to accept up to 4GB SD cards for additional data.