Jeez guys, be patient. Run it on Boot Camp, if you can. Works perfectly fine (of course, since Boot Camp runs Windows natively). I dunno about older OS versions, but I do know that the latest update for OSX 10.7 sped up rebooting into Windows dramatically. I used to never bother rebooting with BC because the wait was so annoying. Now, I can reboot and have my game up and running in less than a minute.
And remember, if you buy the game via Steam for Windows, there is NO CHARGE for the Mac version when it comes out!