This is MacRumors, so let's get OCD for a minute.
TL;DR Summary:
Basically my display has a fairly noticeable brightness/gamma gradient from top to bottom. It doesn't really impact normal phone usage, but you see it as a white point shift. In video, black crush is visible on one end of the display.
After discovering the gradient I found threads complaining about this issue on previous devices with AMOLED displays.
http://forum.xda-developers.com/showthread.php?t=2463786&page=5
It is apparently a common variance in the display tech, and if that's the case then maybe it's worth it for the incredible black levels and color gamut? Both of those aspects are very impressive.
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I was testing out the device yesterday and noticed that it seemed a little bluer towards the bottom. I'd seen this on a store display model too and at both times I wrote it off as white point variance, which you'll also see on most IPS panels.
Later in the evening I was in "inverse" mode (white on black) reading some forums when I noticed that the dark grey boxes that were part of the forum interface (would be light grey in non-inverted mode) were disappearing in the bottom 3rd of the screen. Uh oh, time to go to the display tests.
What I found is that the gamma and brightness were both different in the bottom 1/3 of the screen. While all squares were visible on the lagom.nl black level test in the top half, towards the bottom I'd lose as many as the first 9-10 shades of dark grey. It wasn't just black crush as the gamma was clearly higher (darker) on this end as well. High res images (like the ones included on the phone in the gallery) confirmed that near-black detail was being lost in the bottom 1/3.
This is extremely obvious with solid color backgrounds. Doing fills at various grey levels on Sketchbook makes it pretty obvious if you have a significant gamma gradient.
Before I go any further I'm going to repeat that this is not a big issue for any kind of normal usage. In general what you see (on white) is a slight color shift which looks a lot like the variances you can get in LCD panels. The only reason I'm even looking at it this closely is because the OLED black levels are so impressive (better than my Pioneer plasma) that I might actually watch video content on this screen, something I've never done with a phone before. As a result it's getting the same treatment I'd give a TV.
So here are some images. First you can see what looks like white point variance on a bright screen (of course on camera it's more apparent that this is a brightness variation; I didn't initially pick this out as a brightness issue):
Image
And now a dark grey which highlights the black crush:
Image
It's not unlike something you could see on a plasma TV actually. A high end PDP wouldn't have that much variance and it would be a peak brightness that changed instead of gamma, but I just think the fact that the panels have a similar type of variance is interesting considering the similarities between the display technologies.
I can't say anything about how common this is, and I'm also not suggesting that it's a good reason to exchange (as I mentioned the effect doesn't bother me in normal, non-video usage). The display is otherwise quite impressive and I've really been enjoying having access to a Wacom digitizer at all times.
Basically take whatever you want (including that I'm being obsessive) from this. I just felt like presenting the information in a place where over-analyzing details is generally accepted.