I'm a hard core gamer (FPS) and I find Linux to work just fine for me. I play all the Quakes, Dooms, ROTT (Cant even play that one on Windows), UT2004, MOHAA, and tons of others NATIVELY.
I play Half Life 1 and 2, American Magee's Alice, Far Cry, Diablo 2, Fallout, and some others using Cedega and they all work great. Cedega has a FAR better interface that using plane 'ol Wine, but it will cost you; $5 a month.
Gaming is not ONE of Linux's strong points, granted, but a hard core gamer like myself gets by just fine. I have not needed Windows for years now.
For the record, I have 2x GeForce 7800GTX (SLI), AMD X2 4400, 2 gigs of ram and an Audigy 2.
Linux plays native games FAR better than Windows does (On my box anyway). Games that run natively are much smoother in Linux; Windows might edge out in FPS scores (Very slight lead I might add), but its too jerky in Windows.
YMMV, however, depending on your hardware. I made it a point to get top of the line hardware so I can enjoy top of the line games while in Linux...
Joe