Well, I don't bootcamp my MBP (been lazy about updating the firmware on the SSD), but here's my thoughts on the matter.
I've had to quit steam about twice - the first time it just stopped responding, the other time it claimed that I was running a game when I first tried launching Portal.
Portal itself isn't too hot - haven't tweaked any major settings yet, but 1600x1000 is the best rez I can get out of it. 1900x1200 causes framerate drops. And (from what I can tell) the OSX mouse acceleration isn't helping the game much, either. Feels klunky, but I'm sure some updates will get pushed to help polish it.
That's the bad news. The Ugly News is that for whatever reason it doesn't consume more resources when settings are raised - hl2_osx tacks out about 35% for me, and Steam at around 15%. Could be the idiot that I am is running it with VMWare in the background, but still my processor isn't getting used 100% (more like 50% on each core), so... 😕
The good news? Actually is playable as compared to running it in Crossover or *snort* VMWare. Bonus: higher resolution.
Steam for Mac is a keeper.