I'm about 98% in OS X, and 1% each in Windows 7 and Ubuntu Linux.
I was lucky to get a full retail copy of Win7 through work (because I definitely wouldn't have purchased a full $300 license for the little time I'm actually in Windows), and the only reason I have it is to simply keep up with the new features and keep me current with the OS for my customers.
I loved Windows XP - thought it was the best OS Microsoft had put out, and Vista ultimately made me switch. I'm glad, because once I went to OS X, I realized how much easier, intuitive, and more powerful/stable/secure it is than Windows.
Linux is, in its own way, very similar to OS X (and I've used it on and off since the early 90's), so I like keeping up with that OS as well.
All my day to day activities are done in OS X though, and will be for the foreseeable future. The only way I'd ever consider going back to a PC is if Microsoft finally wised up and built their OS on top of some variant of UNIX and got rid of that damned registry.