Well, being a GAME PROGRAMMER myself, and working on an XBOX/PC title, I can tell you that code for the XBOX and code for the PC are VERY closely related and only contain a few minor differences.
Gearbox did CounterStrike, only the #1 online FPS, so I'm not worried about the quality. Westlake is an accomplished porting stuidio and I'm sure the Mac code will be done amazingly fast.
They have a year to get it out. That is plenty of time to tweak it up and add some really good multiplayer features.
I don't know about you folks but I play Halo splitscreen DM all the time and have a f'n blast... I'm not worried about the PC/Mac versions being fun.