As much as people complain about the Mac's lack of performance compared to Windows (which is true, DirectX is king in so many ways) I played through Portal quite happily on my Geforce 9400M-equipped Mac Mini a few days ago. Pretty smooth, and totally enjoyable. Go Steam!
I'm glad Valve is doing a good job beta testing OpenGL on Macs before letting it loose on Linux. *takes cover from handbags being tossed by Mac users*
I saw an article saying that games run slower on macs than the an equivelent PC spec due to using openCL rather than DirectX. There was also reports of some glitches in games like textures not loading and you can't see through the portals
Until Steam has proper games on MAC such as Battlefield Bad Company 2 and modern games for example then there's really won't be much competition. However when there is there still won't be any competition because MACs come with weak graphics cards. I really don't see the point in them to be honest. I think since Apple Macs are becoming more like proper computers these days i.e. since Intel chips were implemented I think that Apple should just release Mac OSX to PCs and start real competition.