Like Parapunter, at the moment, I also only use Vista on a PC that is functioning as a "media centre" (i.e. playing videos + music via TV and stereo).
However, I don't like it. Lots of annoying features - many of which you can turn off but of course you then lose some functionality/security. With file sharing for example, getting a balance between security and functionality was tricky.
I've been a Windows user for a long time. 3.0 > 3.1 > 95 > 98 > 2000 > XP. And NT 3.51 > NT 4.0
This is the first time I've been disappointed with a new version. Same story with Office - I think I'll give 2007 a miss for a while.
After over 15 years of rushing to upgrade to whatever Microsoft had to offer I'm finally getting off the train and looking elsewhere. Seriously considering using Linux for my media centre.