I use all three OSes at least once a week... and (usually) both Windows and Linux daily. Well, sorta. I only use irssi on the Linux box.
(And usually only iTunes on the Mac.)
But, I've used all three OSes as my daily use, main computer OS.
I prefer USING OS X - it "just works" - but I use Windows as my main OS. Why? Because it almost "just works," and OS X on my ThinkPad
doesn't "just work." And, Linux... it's made quite a few strides in that department, but... I've never had a Linux desktop "just work" in the long term. Eventually, I'll want to install some app that isn't in that distro's package repository, and even though all of its prerequisites are fulfilled, it won't build, no matter what I do. Or, I'll want to do something that the distro doesn't support, and all the HOWTOs I can find error in ways that it shouldn't. (I recall having major issues using a shared printer with Samba on SuSE 8.2... eventually, I found a HOWTO for a really old version of Debian that worked - but the HOWTOs for SuSE, RedHat, Mandrake, etc., etc. didn't even begin to work.) That said, I have found that the distros that I've had the best luck with are always the distros that are the distro du jour - so, right now, that's Ubuntu. Both because, people are targeting packages for it, and people are writing multiple HOWTOs for it. But, still, I've run into frustrating dead ends.
The odd thing is, with lots of open source software, I run into these dead ends - inaccurate HOWTOs I think is a lot of it. I've even run into some of those dead ends on OS X, with various OSS software. (Not that much on Windows, though.) And, no, I'm not compiling from CVS, or running nightly builds.