Macs have always struggled as a gaming platform, as the limited market is not particularly appealing to game publishers. The recent introduction of Steam for Macs has had a positive effect, however. There's now a much wider selection of excellent games available for Apple devotees. This, combined with the decent GPUs that Apple has been putting in most of its systems, makes a Mac seem as if it could be nice system for a casual gamer.
The performance, however, doesn't measure up. According to benchmarks, Mac OS X systems are significantly slower than comparable systems running Windows.
There's two reasons for this. One is driver support in the operating system itself. OS X obviously does not support DirectX, so that leaves OpenGL as the only option. OpenGL support in OS X isn't especially good, however, and Apple hasn't been updating it aggressively. This leaves Macs with a fundamental disadvantage compared to Windows.
The second issue is support from developers. They have limited resources, and while Steam has made it easier for them to sell to Mac gamers, it's still not the top priority. Even major developers like Blizzard are often slow to respond to complaints made by Mac users. I could not play Starcraft 2 on my MacBook under OS X when the game was released due to graphical glitches, but the title ran perfectly when I booted my MacBook into Windows 7.
Clearly, Windows has the advantage here.