Well, I’ve been a Mac user for over 4 years now, and I can definitely state that I’m not going to go back to windows (any version), so far !
But there is one thing that I miss : windows games. I’ve bought myself an xbox with a seperate screen, which is fine and sometimes better for playing games, but some games only come out on windows, like Team Fortress, Crysis or The Witcher or S.T.A.L.K.E.R. or Broken Sword and those are all games that I like playing (whenever I can find the time which is, frankly, not a lot).
I bought Parallels Desktop for Mac, hoping that I could play windows games, but they seem to have stopped developing for dx9 games compatibility, and their updates have been become very slow. So recently, when Codeweavers announced Crossover Games for Mac, I snapped it up and have been using it since. Admittedly with mixed results, but boy when it works for a game, it works well !
Crossover Games For Mac is a specialised version of Crossover (which is based on Wine, the not-an-emulator of windows that recently reached 1.0 status) that has been fine-tuned and built for Mac. Of course you can install Wine yourself, but on a Mac this is not exactly an easy thing to do. Especially the OpenGL stuff seems hard to get to install (at least this is where I got stuck when I tried).
Codeweaver (which is a major sponsor of Wine) has list of games for which it is compatible here. Guild Wars, Eve Online and Half Life 2 are all pretty major games, and these and quite few more from STEAM are guaranteed to work.
Highly recommended if the games you want to play are officially supported by them. There is also a compatibility database that you can check for all the other games in existence.
I’ve gone one step further, and have become an advocate for testing 3 of those games (Broken Sword : Angel of Death, Jade Empire and the Witcher) against each new beta. So far, none of them have started magically working, but I keep hoping…