Took a late lunch so I could go home and have a few minutes with my Paperwhite today before coming back to work. So here are some very quick, fifteen minute impressions from using it.
I have one "pinhole" light leak in the lower right quadrant, which was really noticable in the setup screen when you first power it on because of the graphic they use (with the person under the try reading the book and the hill is nothing but black). Not sure how noticeable it will be in regular use yet.
There is definitely some "spotlighting" near the bottom of the screen when you have the light on bright. I doubt this behavior is much different from unit to unit since it's simply a result of the light being brighter where the LEDs are at the bottom before the light refracts enough to even out over the rest of the display. At least the LEDs are on the bottom on the PW though. The light really does improve the contrast and it's a nice neutral "paper tone" to the light that's nice - I hear the Nook light is sort of green (or blue?), so that may be an improvement, I can't say. However, even with my Kindle Keyboard I rarely used the built-in book light in my case, I prefer to just turn on a lamp or something to read because I really like the look of a standard e-ink screen (lower contrast and all), so I'll probably tend to leave the light off or very low. Even having the light on the lower settings noticeably increases the contrast without it obviously being a light (if that makes any sense), which I found an interesting thing to see. So I'm likely to leave it on the lower setting all the time - the spotlighting is also less noticeable at lower settings. It may be my eyes fooling me though, but it seems like the PW's screen is lower contrast when the light is completely off than the old Pearl E-Ink screen. Perhaps the higher resolution e-ink displays just aren't as high contrast yet. This could be my imagination though, so don't take it as gospel.
The resolution upgrade of the screen is very nice. I'll check tonight with the comic sample I downloaded yesterday and see if there's obvious visual improvements reading a comic on the PW versus the lower resolution Kindle Keyboard. However, the latest software upgrade to the Kindle Keyboard noticeably improved the font rendering on that device (to my eyes anyway) so I'm not sure the text rendering is all that much more impressive at larger font sizes. If you tend to use one of the very small font size options the resolution improvements will probably be more noticeable.
The UI is very responsive and impressive IMO. I've not used any other touchscreen e-readers so I'm only able to compre it to my Kindle Keyboard and it's night and day above that. I really fell in love with it - and that's just my impression of the responsiveness and rendering of the homescreen. I haven't even tried looking up definitions in a book or doing the X-ray stuff yet. The on-screen keyboard, in particular, is very impressive. It's actually more responsive than the screen itself, so if you go really fast you may not see the keys blink while the device will still register the key press. This seems like it could be disconcerting until you get used to it, but I really don't see how anyone could prefer to use the older physical keyboard to this, it's definitely faster using the on-screen keyboard than the physical one from the Kindle Keyboard.
All in all I really like it, though I'll probably give it a day or two to hear more impressions and see if I want to try and get a replacement free of pinhole light leaks - I only have one, so I'm going to wait and see the quality in that regard with the shipping units to see if its worth bothering since right now it's not very noticeable.
