I think the screen gradient is there to some extent in them all, but there is a lot of variance, and it's not just down to differing eyesight. If it bothers you, try an exchange. I did that when I bought my PW (I felt it had a blue tinge to the screen at low lighting conditions), but when my replacement arrived it was a lot worse, and I realised that the screen on my original was actually pretty good.
I've attached an image of my Voyage (sorry it's not the greatest pic), and I can clearly see a gradient in low lighting or when reading in the dark, but it's a 'smooth' gradient from bottom to top (and to my eyes, not too bad). If one side was lighter as well, I don't think I'd be happy to live with that, refurb or not.
Incidentally, contrary to jswinden's view, I think its the glass screen more than the PPI that makes the text look much clearer on the Voyage. When I look at the likes of the PW3 & Kobo Glo HD I can only really notice how much better the text looks when I see an older ereader next to it. With the Voyage, I can look at it on its own and see that the text just looks better.
