I disagree.. the DirectX->OpenGL gap is narrowing; it could actually be closed in a couple years. You can also throw Windows on it if you really want DirectX performance. This won't give you exactly the performance of dual desktop GTX 680's, but it will scream. If you're willing to sacrifice about 10-15% performance for a better all-around OS, an iMac with 680MX will be a fantastic choice.
I'm assuming they have a heat solution; if they don't, they'll be hearing from me quite often. ;-)
There's nothing to disagree with.
Gaming and iMac don't mix. Hey I tried for years, but ultimately it just makes so much more sense to build a Windows pc for gaming and buy a Mac laptop or Mini or MBA for OSX. And you'll still spend less money. And your gaming pc will have faster hardware and be upgradeable if you need a new gpu or SSD or .... And you'll have 2 computers. And you can then have a portable Mac if you want with a gaming pc at home. ...
It isn't like you can play every game in OSX either on your iMac so you'll be booting up to Windows. Never mind that games in OSX run slower on the same hardware.
Gaming on an iMac is ultimately for sadists. It's a universal law. IT is what it is. Like gravity.