Have been playing Marathon and the total conversions for a couple of years now. Like a lot of people, I got hooked shortly after playing Halo. I had a Mac since I was five, but had missed out out the awesome original Marathon games!
Not an expert on FPSs or anything, just like a fun game when I can find one. I especially like playing co-operatively with other people in the games, which is one of the reasons I love Halo and Marathon!