I have a Late 2012 27" iMac (3.2 i5, 680MX GPU) and I love it.
It is near silent, even under load, runs very cool and has plenty of CPU and GPU grunt for whatever I throw at it (gaming in bootcamp mainly).
Even under heavy gaming the temperatures are nice and low - around 70 C with stock clocks (It has plenty of overclock headroom, but I don't bother since it is more than powerful enough for me as is).
The screen is excellent, and if you sit at an average distance away from it (around 33") then it's physically indistinguishable from a retina screen unless you have razor eyesight (that's simply physics). If you are closer to it then you will be able to tell the difference, but even then, it's an outstanding screen.
By all accounts the major downside of going for the retina iMac is that it runs hot due to the GPU they stuffed in there. The 680MX I am running is cool and quiet, as is the 780M in the current non-retina (they're basically the same card). The AMD card in the retina runs hot and causes throttling issues if you push it hard.
If you never intend to work it hard then the retina screen may be worth the premium to you.
Thank you for the detailed info. Unfortunately, Apple just removed the non-retina! My wife plans to lightly game on it, so I doubt that we will push it too hard...