There is something I don't get… Bitcoin is all about freedom, free software, geeks and everything, so why do we have windows users here?
Can't really speak for others, but:
Don't really hold any OS religiously here. In fact my main computer is dual booted with Linux and Win7.
For me, it depends on what I'm doing, if I'm programming/doing crypto stuff then I typically boot linux. However, I am also a part time gamer, and really many games just don't support linux (and the ones that do don't usually do it well).
I don't use Mac mainly because I have no need for one.