I tried Vista on Parallels. I really tried to like it, but I just couldn't keep it running. I went back to XP.
Now I am trying Windows 7 on Parallels. And I am impressed. It runs stable, it runs smoothly in a VM and (so far) plays nice with the virtual environment. No aero in the VM, but that is of the least importance.
Being that I am usually on the other side of the argument re: MS/OS X or MS/*nix, I have to honestly admit that so far I really like Windows 7. Mostly because it works in the environment in which I use it (Parallels) and works at least as well as XP. I have not had any of the temporary freezing issues with 7 that I have had with XP, so at least in that regard it is working slightly better.
"I just want to be a good engineer." -- Steve Wozniak, co-founder of Apple Computer, concluding his keynote speech at the 1988 AppleFest