I agree with what most people have said, how much of a gamer are you? Obviously, its not gonna run as smooth as a desktop running the latest and greatest video card in SLI. Then again, this is a laptop we are talking about, its less than an inch thick and its mobile. With that said, I would have to say it's pretty damn good.
I play COD4, HL2, Team Fortress 2, Age of Conan, and Fronlines on my MBP. I have "some" complaints because I'm use to having a desktop with the best hardware. You're not going to be able to set some games at its highest settings so you're not gonna get the best quality(hence "my" complaint), but its not terrible. You will have to do a lot of tweaking to the video settings in each game to maximize your appetite for quality and performance.
All in all, I'm pretty happy with it. I am able enjoy my games with pretty good quality and performance...and it fits in my back pack.