I'm playing on a mac, as you might have guessed, but the client is still Windows only (Mac client is in the making). So thank God for Bootcamp!
Download and installation was easy. Only problem I got was that the map server was down on my first day. When it is down you can't download the maps, and so you can't play unless you already got them before. Why can't they include the basic maps in the install ? It was around 1.7GB already; a few more MB wouldn't have mattered much.
Anyway, the game itself is great! I got my ass kicked three times in a row —all on Novice level as well— but it was fun. I finally managed to win one game with a combination of Stalkers and Coloxen, with a few Immortals thrown in for fun. (Yes, I prefer Protoss.)
Also, if you like playing with a pen tablet instead of a mouse (as I do), you'll be fine. With the old Starcraft this was a bit of a pain, but in SC2 it wasn't a problem at all.
The replay feature is also very nice. It showed me that I was actually quite a bit slower in my build than my opponents. Timing is very important in this game.
Definitely will be playing more.