Asides from WoW and maybe the occasional bout of Warcraft III, I don't play much on the Mac. I do have KOTOR, XIII, Fable the Lost Chapters and Uplink that have been ported to Mac, pretty well too. But I've tried to game more on my Mac with mixed results, especially since I also had some old games that now only work on Power PC macs.
The thing I found out with Mac versions of the EA games of late are some weird emulation type tech they use. Basically they wrap up the Windows x86 code and translate it to Mac x86 in some form. Thing is though, the requirements are steeper. No optimisation for Macs at all. In fact come to think of it I think they use Cider to make it work. I've even seen some (hacked to bits) homemade cider ports of other Windows games.
If you already own a few PC games (like I did before I switched) try out Crossover, it's an app that runs the code WINE style. I used it for my half life 2 and various other PC: http://www.codeweavers.com/products/cxgames/
In the end though if you want native level performance for gaming on your Mac, just set up Bootcamp. Just been updated to support Win7. I had my old WinXP license that I just used for it, and the games ran as per normal. The bright side is then you have the best of both worlds, none of this 'OMG wai u want to gaem with max dey so ghey just buy a peecee' nonsense.
Anyway there's my long winded two pence about it.