I'm bootcamping Rift on my iMac (vintage one, at that). It works, if you don't mind the low quality renderer and rare (maybe once a night) DC from overload if there are a couple hundred people together at once... but if you're getting something for the express purpose of running Rift, it's not what I'd recommend (sadly, as I've been a Mac guy since there was a Mac). New iMacs should run it reasonably well, I expect. But Booie's right, they're not gaming machines. And iMacs are not upgradeable, so as graphics cards advance, yours doesn't. :-)
Now, if you can afford a Mac Pro (starting at $2500 last I looked), it's a different matter. You can upgrade your components in it.