I bought the Paperwhite and love it! Except for some very slight "shadowing" at the bottom of the screen, the lighting is extremely even and pleasant, and there's more than enough of it. I keep my screen turned down a few notches, and notch it down a bit further when reading in the dark.
The "shadowing" isn't really shadowing, but just the areas between the four LEDs as the light begins its trip up the screen. It spreads out quickly but not instantly, leaving ever-so-slight not-fully-illuminated patches at the bottom, down below where any text resides.
It's really, really minor. Overall, the lighting is more even than anything I've ever gotten from an external light.