I still use Windows XP Pro SP3 on my main home PC and at work...just reliable, stable and supportable in the enterprise moreso than the others IMO. I have Vista Ultimate on my gaming laptop but truthfully I purely use that machine to play the one game I seem to ever play since getting it..L4D. It's well spec'd and decked out and it's been good and stable, but I only do one thing with it and unusual for me, I just havent been inclined to dig into Vistas bowels and see what it can do. I just turn it on, play my game, turn it off. Kinda a overpriced and underutilized Midway arcade machine from the 80's.
I should be getting my copy of Windows 7 Pro in a day or so but I'm going to virtualize it and kick it's tires on top of VMWare workstation. I have decided to not bother with any upgrades, everything I have is working and I dont need any headaches. Virtualization or/and a new PC with Win 7 on it is the route I've decided to take.
BTW..I just bought a MacBook Pro with Snow Leopard...."awesome"!!! Been away from my trusty Mac's for a few years, glad I brought one back into the family.