Finally, a reviewer that states the truthfrom a normal sitting distance (more than six inches away from the display), the difference between the Retina and non-Retina 27" iMac is negligible and you can barely tell the differences.
Finally? Retina has been around forever and this isn't exactly news if you understand the whole point behind retina. And it's not true, even accounting for vision differences. Don't take my anecdotal possibly biased point of view for it; see the calculator I cited below.
As a rough guide, pixels on the riMac begin to resolve at 16" from the screen for a person with 20-20 vision (same as the MBP 15"). For the regular iMac it's 32". That's a big difference. From what I've read the average user sits 24" from the screen.
At the "best" setting because of the way the riMac displays UI elements it might be a bit different. But the difference is definitely dramatic. I have the riMac running a good 27" IPS monitor right next to it, at 2560x1440. And I've had to do some juggling to make it worth looking at, since I'm now closer to my iMac.
Also, I use a stand up desk. So my Mac is a bit more in my face than it was when I had a sit down desk. I noticed I gravitated to just about 16"; closer (which you can do) is front row theater like. But I also find myself moving closer to see detail, like the postmark on the Mail icon. With my old iMac I found myself leaning back.
It's true that you have to consider ergonomics. You would be wasting your time if you had to sit 36" from your screen. Here's a calculator so you can figure out what distance is "retina," and there are others online to calculate sitting distance for home theater setups.
http://isthisretina.com
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More info on this? I would be getting the Retina iMac for audio production work. I was hoping to get something quieter than my 2010 MBP for this exact reason (I record in the same room as my computer)
On the fan, see the Verge review; guy found it essentially silent using Premiere and After Effects.
I'm wondering if those who say they are hearing fans have spinners instead of SSD only. I've got the fan going right now at 1200 and don't hear anything, but it's not an absolutely quiet room.