I bought my first Kindle in 2009 when the Kindle 3 first went on sale. It was perfect. Since then I've bought every model Kindle except the DX and the Oasis. All but one has been perfect: a Paperwhite with a non-working 3G. Everything other than the 3G was perfect.
I've also bought multiples of most of these Kindles for myself. I currently have a 1st gen Paperwhite, 3 3rd gen Paperwhites, 3 4th gen Paperwhites and a Voyage. I did have a couple of 2nd gen Paperwhites that I've sold to neighbors. All have been perfect except that one with the 3G that didn't work.
I live in a retirement home and most of the time I'm the only one here with internet access so when someone sees my Kindle, borrows one to try out, and wants one, I buy it for them, set it up, etc. I help them open an account, they come here and use my Wifi to buy books. I help them learn to use it and when they have issues. I've probably bought 15 Kindles for my neighbors since I bought my first Kindle. All have been perfect.
It is true that if I put my various Kindles side by side their screens all look slightly different, even among the same model. That was also true when I was using Palm Pilots as well. I put them side by side and each perfect screen is different than all the other perfect screens. I suspect this is the nature of screen making. They're probably made in large batches and each batch is probably slightly different from every other batch. I'm just guessing at that. I do know they're all a bit different.
The blotchiest light I've seen on any ereader is on my 3 Kobo Auras. The original Auras. I bought 4 for me and sold one. I bought half a dozen for other people. All had blotchy screens. When I compare them to any lit Kindle they're kind of ugly. And yet when I'm reading a book I never notice it. I'm in the book looking out.
A similar thing happened in the early days of high fidelity sound. I don't know how old others reading this are but I listened to music for a lot of years before there was such a thing as a high fidelity speaker, much less stereo. Then Hi-Fi came along and it was wonderful. But not necessary. Just really, really nice. I was still able to enjoy music on my old fashioned record player and radio but I like the Hi-Fi more.
And then people began comparing the various speakers and amplifiers and tuners and suddenly whichever one they loved was wonderful and the rest were garbage. It had nothing to do with music. It was about vanity!
Barry