Still using XP on my laptop, runs nice and fast and does everything I need. No need to change it really.
I have installed both on my PC, windows XP as well as windows vista. But normally I am using windows xp I don't like vista.
XP is as stable as Mac. Although macs more secure, can't be arsed learning how to use a Mac plus they are more expensive and I don't like how everything is integrated in the monitor.
I use XP still now. I was in Vista Beta testing and I hate it.. Eat lot of memory for no good things.. I love XP
I'm still using XP too, my sister's laptop has Vista and I've used it a few times, but its not as good as XP imo.