Running/Testing Windows Vista Ultimate on my back-up Pc (Athlon 64 3000+, 768mb ram, Radeon 9800XT 256mb). So far so good. Haven't played games on it though.
One comment though, the new interface (aero) didn't impress me. Probably because i have a mac. The 3d window effects really mimics OSX. Just plain rip-offs. lol. I really don't see the rush in going to Vista, as of now. However, I wouldn't say Vista is a failure. Eventually, people will be forced to go with Vista especially gamers. When Vista only and DX10-native games come, they won't have any other option but to go with the flow. ^_^