What do you find problematic on the Mac version? My office has weekly TF2 games and a bunch of us use Steam on OSX. The only gripe I have is that it doesn't remember my password, but other than that it seems pretty much identical to the Windows counterpart.
Personally, I'm impressed by how horribly implemented Steam's OS X gamepad support is. That takes skill to do, when other apps Just Work™.
Also, Steam.app used to suck heaps of CPU just idling in the background, often enough that to make the fan audible in laptops. That improved lately, but still isn't perfect.
Not to mention the frequent crashes when you quit. I guess it's quitting either way, but still.
