Rare? You haven't even a clue.
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I'm with you on this. I've had three iPad 3 and two iPad 4 and a few friends of mine own Retina iPads as well. Every screen looks a little different, some are yellower, some are blue, some have a tendency to show more red. That's not that bad, slight variations are to be expected on consumer-quality screens.
What really bothers me is the uniformity - or rather the lack of it.
Both my iPad 4 were nearly perfect in terms of calibration, showed my pictures (I'm a photographer) exactly like they should. But when looking on a white screen, surfing the web, reading or browsing the App Store there's always this green-to-pink gradient, top to bottom. It's not just one iPad. It's every iPad. The perfectly calibrated ones are the worst: you've got a brilliant screen, showing crispy whites right in the center of the screen, emphasizing the green and pink tint on the top and bottom even more. Blueishly calibrated screens are better, because they tend to mask the problem a little, showing a little variation in brightness but hiding the color variations.
I'm still not sure what to do, I have another week to return my iPad 4 but I'm disappointed that, a year after the introduction of the Retina display, there a still major quality problems with the screens.
Someone already pointed out that the majority of people don't notice or don't care about this problem. That may be right. It's still sad for people like me that are sensitive to such color variations.
Aside from that, just nice to know: there have been a lot of discussions in tech/photography forums about a green-to-pink tint problem with LG IPS-panels on semi-pro grade displays that still cost a few hundred bucks. It's obviously the same problem on most iPads today, indicating that Apple may have switched to LG as their only or at least main supplier for 9.7" iPad Retina screens. iFixit found an LG-panel in their iPad 4 teardown as well.