went from my loaded PC gaming rig (i5 2500k, Radeon 6970 2GB etc etc) to a 13" cMBP.
For the most part I can play the same games I used to play. Admittedly I was a little worried about going from a dedicated GPU to an integrated 4000, but it handles itself quite nicely.
Games I have tried out on the Mac:
League of Legends
Diablo 3
Starcraft 2
Bastion
Lord of the Rings Online
The 13" games adequately, don't expect mind blowing fps, but it wasn't a big disappointing going from a fairly high-powered desktop to a much more compact solution.
I have yet to try gaming in bootcamp.