I can play WoW on my Mac, which is the only game I've been running on my Mac and not once has the game hiccuped..
Look, it's more along the choice of how much you want to spend and what you are going to use it for.
I love using my mac, but not being upgradable is a huge problem, especially when in 3 months something better comes out and the only way of getting it is to buy the computer all over again.. instead of just buying a processor & motherboard...
But OS X is completely amazing!