Don't know anything about the Onyx & Cybook but in terms of the quality of the screens alone, the Nook appears to be the least impressive out of the Nook, Kindle, Kobo (screen not quite as clear, leds are at the top instead of the bottom which can make them more noticeable). Probably down to theirs being the first to be released, the Kindle & Kobo have improved things a bit with their subsequent releases.
The Kobo has the most consistent, uniform screen, and it uses a grey band along the bottom to further hide any shadows from the leds, but if you are light sensitive you might find it has a slight bluish tint and is a bit too bright.
The Kindle appears to have got the quality of the light slightly better, its maybe a bit warmer than the Kobo? However, you have to get one with a 'good' screen, otherwise you might see blotches and some shadows from the leds. There is some variation between different Kobo's but not as much as there appears to be with the Kindle.
Edited to add - the fact that most people who have bought a lighted reader wouldn't go back to a non-lighted one (see thread in general forum) should tell you enough. Most agree they are far better than using a standard e-reader with a clip-on light, and its really only if you are expecting LCD light quality that you will be disappointed (though there are of course exceptions!).
Last edited by soulfuldog; 02-05-2013 at 01:05 PM.
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