I run it because I need to support it. I know XP better than the back of my hand, I can walk people through whatever needs to be done from memory. I've been able to do this for a long time because I started using XP as my full-time workstation when it was RC1. Now I need to know Vista, so I run it at home. Because I don't like to wait, I built a system that doesn't make me wait for anything. Yes Vista is slower than XP and 2003, but not by as much as everyone is saying.
It seems that everyone on the internet needs to have an incredibly strong opinion about everything. Black and White. This is good, this is bad, and that is evil. It simply isn't true. 2003 is faster, Vista has more features, XP is more compatible; they are all fine for 95% of users. 90% of users wouldn't even notice!
I'm not saying Vista is better in all cases for everyone, or even better at anything for anyone, I'm just saying that you won't go to hell if you are using it. This sensationalist BS is very tiring.