I got my iPhone 6S Plus 64GB space gray (running iOS 9.1) a couple of weeks ago and it has two screen issues:
the first one is non-uniform backlight, which is very well depicted on a photo that someone else took:
http://forums.macrumors.com/attachments/pasted_image_10_3_15__4_10_pm-png.589387 /
Regarding this photo, mine has the exact same problem, with addition that there are 10, not 9 such stripes - one is also at the position of battery indicator, which is also seen on this photo, but the user obviously forgot to mark it with an arrow.
The problem is very annoying on white backgrounds, but also noticable on some others too, e.g. violet. Resetting the phone, turning the white point off, increasing screen illumination or reducing transparency don't help noticeably, so it's obviously a hardware problem. I'm also quite sure that future iOS upgrades won't help.
I have also inspected the iPhone 6S space gray of a friend of mine and it has the same backlight uniformity issue along the top side of the screen, perhaps just a bit less pronounced.
Considering the length of this thread (and also other quite numerous threads that I found on the Internet regarding this issue) and based on my own experience, I can say with fair confidence that the vast majority of iPhones (6 and 6S as well, both regular and Plus sized) are affected by it. It's not random, it's systematic. I'm also quite sure that it should be quite hard to find the iPhone 6/6S with perfect display, by "perfect" meaning that there are no such glitches, very obvious and very disturbing even to my non perfect vision.
The second problem with the screen is very slight (but also quite annoying once you notice it) blue tint along the screen vertical edges.
I still have my old iPhone 4 which has a perfect display as far as I can see.
Best reagrds,
Albert