I run XP SP2 daily, but I'm sort of a hybrid Win/Linux user: WRT on my router, SSH often, write code for a living. Rock CYGWIN daily. Sometimes I go hours of without realizing that my wireless mouse is out of juice... 99% of features I use in 90% of my apps are bound to keys.
I've tried WINE, dual booting, etc, but I find no reason to switch (or even boot occasionally) to Ubuntu. There's nothing I can do (being a Linux non-guru) in Ubuntu that I can't in Windows. In fact, I'd venture that with my uber customized XP setup, I can match most Linux power users in raw speed.
Sadly, however, I know I'll have to switch to either Ubuntu or OSX with my upcoming i7 build: I'll need all 64 bits. Windows 7 is a distant possibility, but unlikely. Is there a guide out there for people like me?