Unlike other people, I rather stick to Vista since I don't see any reasons for me to upgrade. Vista actually run pretty well on my systems. Mac OS X is better, but on my desktop PCs, Vista isn't bad. Does almost everything I can do on a Mac and Linux + run my games.
Probably, I'd only shove Windows 7 into my media centre because native H264 playback on my media centre would be nice instead of relying on those third party filters that doesn't integrate with Windows Media Center well.