I was debating between the D900, D600, K800 and the W810. For me personally, my list of priorities for a phone:
1) reception
2) camera quality
3) physical appearance
4) multimedia (mp3 player, video, etc)
I've been reading a lot of threads on how the 850mhz RF reception wasn't too great on the D900/D600's so that immediately took them both off my list, especially considering how expensive the D900 is. They also take microSD memory which is super expensive and super small!!
The K800 (triband 900/1800/1900) was a bust also because it isn't quadband. They have the K790 (same as the K800 physically) which has 850/1800/1900, but in the end it's still a triband phone. If I'm paying that much for a phone, it better be quadband with great reception, play mp3s like an ipod, and take high quality photos. If it were quadband, I think this phone would have been perfect for me. K790 would be good if you never travel out of the USA and the K800 would be good if you don't care about 850mhz RF roaming. I didn't want to compromise...
I ended up getting the W810 because it is quadband, has good reception, great mp3 player, uses MemoryStick Pro Duo (affordable and common!), and has a decent 2.0mp camera, aaand it was cheaper at around $300 shipped overnighted.