I'm a Mac user of Steam, only having dabbled with it on the Windows side briefly, and have been enjoying Portal. I haven't noticed any performance issues on my 2009 Mac mini, but then Portal obviously isn't too taxing on the hardware. I've also got Torchlight and it plays great too. I own Team Fortress 2 on Steam (that's what I briefly dabbled with), so I'm anxious to get a chance to try that out more extensively on the Mac side once they release it. I suppose I might run into performance issues there, but hopefully not.
I also picked up the Indie Bundle but haven't really played any of those games much yet.
EDIT: though Steam itself is a bit buggy and I've had some crashes of Steam (not a game yet, just Steam). The in-game Steam overlay feature isn't working at all for me - pressing shift-tab does nothing at all in spite of the invitation to do so at the start of every game.