Is a Mac good for playing casual games?

I hear all sorts of "hardcore gamers" dissing on macs, but I'd like to know. The only games I ever play on my computer anyways are the Sims 3 and an online fantasy MMO called Mabinogi. My dad looked online and built a Mac with comparable specs to the PC desktop I'm thinking of getting, at about the same price. I'd just like to know if the Mac is going to stand up to the Sims 3 and one online game.