Hello.
I have the current highest spec 27" iMac, and am a giant gamer gear-head. This comp is the first one I've ever owned that I didn't build myself.
The GPU (6970m) is actually an underclocked 6850. It's set to 680 MHz by default, and on that clock it runs BF3 full screen, native res, max settings with zero hiccups ever on BF3 and Skyrim (I game an embarrassing amount and I've seen nothing except a few very minor framedrops on huge events in wide open spaces in Skyrim, never anything in BF3). I'm assuming the reason other people have had some troubles max settings on Skyrim is that it's a texture-heavy environment and they prolly have the 1gb VRAM model. I got the 2 GB and it's smooth as butter. (VRAM being the limiting factor in texture display)
For higher draw games - the most obvious example being Witcher 2, to run on high settings (no ubersampling of course) at native res (1440p) you have to jack up the clock to around 810-820 (IIRC 780 is the native 6850 clock). That's zero problem for this card, and the system can easily handle the heat with minor fan overclocking.
That's a lot of rechnical detail, but the main point is that you can go max-settings in just about any game on the market with the current gen 27" high spec, so presumably the next one with the 7xxx gen Radeons in them will be QUITE a bit better (I've heard estimates like 40% performance increase, and I believe it)
TLDR: Buy it. Don't listen to people in threads like this who make assumptions without looking into gear tweaks which are incredibly easy to do in Windows 7, where you'll be spending your gaming time.
(FYI for other hardware tweakers - MSI afterburner is the most compatible overclock suite for this particular card. Best fan solution has been MacFan in OSX with a restart into Win7, with fan resets upon full shutdown and reboot after gaming sessions. I only add all this because it took me a GIGANTIC amount of research and experimentation to figure all this out. I'm about 6 months into owning it and it's like day 1, so no apparent danger in conservative overclocks - my heat sensors have never read anything even remotely in the "melt the GPU" range. And trust me. I've melted components a few times.)