I'm still using Win 2000 and will continue until I absolutely have to upgrade. I have and am using Win Server 2003 on one machine and am satisfied it will make a good choice for a next upgrade. My machines, even though they are 900mhz thinkpads, outperform most other people's laptops that I have used including my daughters 3.2 mhz machine with 1Gig of ram, running Vista.
I think if I had to use Vista, I would go insane. After my daughter's machine gets to the desktop you have to spend 3 or 4 minutes shutting off all the crap that wants to "help" and "protect" you. It is maddening.