It's easy. Whoever wants to leave Windows uses Linux.
I did. I see no point in using Mac...
-It has less support than Linux.
-You have to pay for it.
-You only get to choose from one Distro: OSX.
Personally, I've seen plenty of Macs, and they're nothing special. KDE3 with Compiz Fusion kicks the crap out of Mac's window manager. And KDE4 will do it even more. Why pay for an overpriced Mac when you can get an even more stable OS like Linux?
8 Reasons Windows Users Don’t Switch