Most MAC games are DATED at best... the new release MAC games are often released long after their PC counterparts.
This might someday change if they get enough market share... but for the long history of the mac, it hasn't.
There's usually a delay of a few months, although some companies like Blizzard do simultaneous releases. If you're a hardcore gamer then a Mac isn't for you, at least not as a primary machine. If you just want to play games occasionally, there's more than enough Mac games to keep you occupied.
That long history is over. Realizing a market of 14 million people who generally have disposable income, they're releasing the latest games not long after the PC version is out. DoomIII and Splinter Cell are not dated.