Windows 7 runs like a dream. Additionally, I should add that I actually thought Vista was a decent OS. Most of the problems people had with it were that they were running them on computers that had no business running it. Vista, especially with the default graphics turned on, required a pretty decent machine. The fault though was Microsoft's for trying to tell everyone to upgrade when that wasn't a good idea.
It's actually pretty similar to what's currently going on with iOS4 and iPhone3G's. Apple has promised that iOS4 runs great on a 3G, but my experience (and that of a lot of other 3G users) is that it severely cripples the phone.
Anyway, your reasons for wanting a Mac are valid ones. If you prefer the feel and logic of the OS, go for it. However, if your issues are simply performance-related, a new computer running Windows 7 will easily do the trick.