'tis the question I've been asking myself the last few days.
I was an early adopter of XP so it's probably been 5 or so years that it's been my primary OS and it's also, unsurprisingly, Fujitsu's standard OS.
I've just noticed that, of my two machines, the WinXP machine is only ever getting used to check my emails and retrieve the occasional file. I could quite easily configure Outlook'07 on my Vista box to do the emails and copying files isn't hard - so it's mostly my laziness that I'm still using WinXP.
Over the last few months I've been getting more and more used to some of the small usability changes to Vista such that I go back to XP and get annoyed that I have to do things the "old way."
All in all, it's that tipping point - do I migrate over to Vista full time and slowly retire my XP box? hmm...
Anyone reading this on Vista as their fulltime/primary OS yet?