I played a couple games last night. I have the quad i7 2.3, with 4gbs of RAM at the moment (waiting on that crucial order). All my test were pretty slapstick, and I only measured perceived performance.
Torchlight- No problems, all settings at max on 1080p resolution. Ran like a champ, but it isn't a very demanding game.
Portal- Pushed it up to max, everything seemed fine. Will do some more test with it when I get home from work today. I'm not sure If I had AA on or not.
The Witcher- 1080p, medium/low settings. Kinda choppy in some of the cut scenes. Actual game world was mostly 30+ fps, but I had never played it before and didn't get far. Worth noting that this game is just a Windows game wrapped in wine, so that may have hit the performance.
Minecraft- 1080p, Fancy graphics, medium draw distance. Ran great, not noticeable drops other than while world rendering and loading into memory.
As usual, most games get a pretty significant boost running in Windows, and if you are a bootcamp user, you can probably expect a 5-10 fps boost.
That said, native OS X gaming on the HD4000 isn't the hell-scape you would assume by reading this forum. Casual gaming is more than doable, and should not be written off completely. The types of games I play, most notably things from humble indie bundles, run very well. Furthermore, less demanding 3d games should run reasonably at lower settings. If you're into emulators, the quad i7 2.3 will crunch numbers like no other, giving a very accurate replication of old consoles. It would be fun to see if this machine could run a ps2 emulator in bootcamp, as CPU is usually the primary indication for emulator performance. I'm curious to try more games, and I have a couple on download right now.