Have a look at the price difference between the i5 and the i7. I went for the i5 as I didn't think speed increase was worth the price difference..
Been running mine now for just under 2 months and no real problems. The screen can be a bit bright straight out of the box, but there is a brightness control in system prefs now to turn it down.
iMac, especially the 27" have had a couple of reported problems. One is screen flicker, not had that myself, but Apple have just released a software upgrade to fix it. The other problem is a yellow colouration to the bottom corner of the screen. Doesn't seem to effect everyone. It's there on mine but it's so minor and well out of the way it's not a problem.I've seen worse on more expensive monitors
Overall it's a good machine.