AnandTech has taken a look at the performance of the Mac OS X Steam version of Half Life Episode 2, you can check it out over here. Mac fans will be glad to see more games available for the platform, but unfortunately the Mac edition is no match for its Windows counterpart. Although the game is definitely playable, the performance is much worse, and the image quality is also noticeably worse than under Windows.
To Valve’s credit, at least on current generation Macs, Source engine games are absolutely playable. It supports Apple’s whole “it just works” mantra. You’re just better off running them in Windows if you have the option. Although I will admit that the convenience of not having to reboot is sometimes worth the frame rate penalty, at least for shorter gaming sessions. If I’m going to be playing for more than 20 minutes, then a reboot is more than worth it.