Ah, so a single user's experience extrapolated to be universal.
I've not had HL2 or portal crashes in osX. I get pretty comparable FPS in both OS.
I'm enjoying them without any crashes with the same settings without having to reboot and quit all my other apps, and have deleted them from my Windows partition.
My recommendation would be "if you're having problems with the osX versions or demand slightly better performance, install them in bootcamp instead."
It also seems that running a game whilst other tasks are going on is smoother in osX (than win7-64). I know a lot of people would tell you not to do that, but sometimes its handy to run games in a window whilst other things are going on.
Certainly on my Macs there just isn't this night-and-day difference some people seem to be experiencing.
I wouldn't say its single user experience - lol... Just check the Steam forums and you will see...
Saying all that I am still on 10.5.8 and I hear that performance is much better under 10.6 - however, still no where near close to performance under Windows / Direct X...
In any case - I couldn't justify updating my OS just so I can play a game with way inferior quality than I can do under BootCamp
So, enjoy your Mac gaming experience and I will my Windows one...