They are not very noticeable at all, less than a pixel I guess. You can see them when looking for them, but while reading they don't stand out.
Today I experienced the screen fading while under plain bright sun. No doubt the strong light and heat interfere with the electric field underneath? But as I've said once: even kindle users prefer to read under the shade of a tree as its iconic image suggests.
I'm enjoying it very much and went so far as stripping the DRM off my kindle ebooks through calibre and now am reading them properly on e-ink.
There's more to read per screen and eyes are very relaxed indeed when compared to smartphone small screen and backlit text...