Heres how I would classify computer users.
Windows - The most mainstream and used. Makes sense to use it since most software is designed for it. XP has been pretty stable for me.
MAC - Good for audio/video stuff, but the average person who gets a mac dosent even do that stuff. They somehow think they are trendy or on a higher plane of consciousness for using a mac.
*nix - Great for servers and that kind of thing. But I think it will be pretty long before joe average newbie is using it as his home os. Mostly anti-establishment geeks who think making money is evil.