For people that want to play games, why not just buy a PC?
Because I want a really nice Mac for a main machine first, and a Windows PC for the occasional Windows game second. I'm not buying an iMac because I want to play a lot of games in Windows, it's just a nice benefit. One that will make me wait on buying a new iMac until I can have a 3TB Fusion drive and still use bootcamp.
To the OP, I've used my 2011 iMac 21 inch with Windows 7 + Steam for a year and a half, and have no complaints. Everything works fine.
I don't think Windows 8 will be officially supported for another 2-3 months.