For anyone who has any concerns about the effect another layer will have on the screen in comparison to the non front-lit e-readers, take a look at the following thread in the Kindle forum: '
New Basic Kindle arrived today'.
Apart from one person, everyone who has posted (and there are a couple of Nook & Sony owners in it too) has said they wouldn't go back to a non-lighted reader after getting the Paperwhite, because the light just makes it so much better.
Yes it does have some effect on the screen when the light is off (the light guide that both the Kobo & the Kindle use also adds a rough texture to the screen), and there's very little chance that the new Sony won't suffer from this too, but:
a) no one really uses them with the light off,
b) the improvement of the screen in anything other than the brightest outdoor lighting conditions negates it anyway,
c) just as both the Kobo & the Kindle have improved on the older Nook Glowlight, there's a chance the Sony will see a slight improvement on the older Kobo/Kindle.