My riMac left me with my mouth gaping wide open after I've set it up and got it running: What an awesome display!
Unfortunately, it had two pixel defects: One stuck one, and one maybe stuck (or some damage in the layer keeping the background light out). So Apple sent me a replacement which I've received a few days ago.
I set up iMac #2 and transferred my data. Wow, no pixel defect in sight ... but ... but ... where's the Oh my god! feeling I have everytime I looked at iMac #1 ???
After fiddling round a bit with different positions, external light drained out etc., I can now say: Display #2 is not bad. But it just doesn't come anywhere close to the epiphany I have everytime I used #1. The display is a shade warmer, especially white just doesn't look as crisp and clean, and grey looks a little bit yellowish. The fonts don't look as sharp.
As an anti-paranoia check, I asked one of my female colleagues to check out the two iMacs w/o telling her what's going on. She's not a tech savvy by far, but it only took her a glance to verify what I was seeing: The second iMac is warmer / yellowisher (she actually called it egg-ish).
All in all: I'll send the second one back. Not sure if I've got the time or nerve to go for round #3 - maybe I'll just live with the pixel defects. On a regular display they'd be unbearable to me, on Retina they're only visible if you go looking for them ...
I think the second display is still OK - but the first one is superb. Sad though they can't push out constant quality ...