I have a PC with Windows 7 for gaming - this is the machine and OS I use most, simply for convenience. If WINE could run all my games, I wouldn't use Windows at all. I dual boot with Kubuntu and run OS X and other GNU/Linux distros using VMWare and VirtualBox. Then I have a hackintosh netbook running Leopard.
All three OS' have their strengths and weaknesses when you're in my position. For most people, one will usually suffice. GNU/Linux is my favourite due to its idealistic community and the principles on which it is based and if there was some kind of OS nuclear holocaust and I had to choose one to survive, it would be GNU/Linux.