My old 2008 mac pro games beautifully. See the specifications in my signature.
The last session I had, I was hosting a livestream in boot-camp (windows 7 64bit Pro) of Dead Space 3, graphics MAXED out. livestream was also recording to my hard drive as we played (unknowingly) along with that I was on a Skype call with the person I was playing co-op with, had all my messengers up, and I have a dual monitor setup so I had task manager running to keep an eye on things
The damages of all that at once? My average cpu usage was 40-45% used, and out of my 14gb ram I had 7gb free yet. The game never slowed down.
Are macs good for gaming? To me, HELL YES.... long as you can upgrade the graphics card as I have done (GTX570 superclocked) Id like to upgrade to a 680 when they come down a bit in price.
My 2008 iMac is still a decent gaming machine. My son and his friends use it for Minecraft, BF2142 and Starcraft 2. Although SC2 to be playable requires low settings.