I am already aware of screens passing through quality control with unnoticed problems since the first two I was sent after pre-ordering had obvious dust behind the screen glass.
I also am aware that a discussion in forums does not in itself indicate a common problem.
However, there are several lines of evidence that, when taken together, suggest there is a more general problem.
1. There is already a discussion on this site about this for the iPhone 6s Plus (in which the smaller 6s is also mentioned)
2. The macrumours discussion (and poll, albeit likely biased toward people who are already there because they've searched for a discussion on that issue) http://forums.macrumors.com/threads/iphone-6s-screen.1921180/
3. The fact that those who've been into Apple Stores to exchange units have gone through several that have the same issue, in some cases without ending up with a satisfactory one
4. A similar discussion in the comments of a 9to5mac post on 6s early issues in general (http://9to5mac.com/2015/09/30/iphone-6s-touch-id-3d-touch-speaker-power-issues/)
Nevertheless, I still wanted to check carefully myself (and as I said that's one of the reasons I started this post). I did not immediately return my unit since glue issues last year for the 6 suggested that waiting for a week might clear things up, but there was no change. I also went to an Apple Store to check out the demo models for myself and noticed similar issues, as mentioned above. Furthermore, I checked for myself that this was specific to the 6s, and indeed found no issue with the 6 demo models, also as indicated above. So this is not just a characteristic of IPS displays being uneven generally (as others have suggested elsewhere).
Certainly millions of these iPhones have been sold and many may not have this problem, and of course there will not be forum discussions about satisfied owners. But the problem is a subtle one that is not immediately obvious (I didn't notice straight away), unlike screen dust, so many people may not report it, while others will accept it and learn to live with it (as stated by a number of people in other forums).
However, it is a measurable problem (others have directly tested screen brightness in different regions of the screen), and it makes the screen tiring to look at because your eyes have to adjust as you read text across the screen. It is also frustrating since so much effort has been made to give the most accurate colour gamut possible, and so on, to these screens
Perhaps things will settle down and this uneveness will resolve itself over time, I certainly hope so. And I shall be very happy to be proved wrong if the next one I try is fine.