Initial setup experience: D. Not sure if it was because I got pulled away partway through and it went back to the sleep screen mid-setup or because the wi-fi signal in our office was poor and it doesn't deal well with bad connectivity. The thing has one button and no manual, and still isn't as intuitive (or anywhere near as responsive) as an iPad, but being a techie I just screwed with it for half an hour until somehow I got it to reset and recover. Had that happened to a normal user with limited time and patience I suspect there would have been at least a tech support call, perhaps a return. But once I finally fixed that, everything was fine.
UI/Controls: C+. I got some previous gen Kindle a while back and returned it in the 30 day return period because it was so annoying. Looked and felt like something 5 years older than it was, and the physical keyboard took up a big chunk of real estate better spent on the primary use case (reading books). This is much improved, with a touchscreen and a single physical (lock) button. Still not as user-friendly as an iPad, but pretty good. Major gripe: it's slow as *$&%. Seriously, this thing is waaay less responsive than my now over 3 year old 1st gen iPad, and it doesn't even have to deal with complexities like color. And I don't think it's the eInk screen, it's the software. Turn a page...wait...do it again..oh! There it went--two pages! But you learn to accept that and it's fine. Getting very tired of accepting poor performance from gadgets, though. Every once in a while you step back and realize you've been trained by the device and that someday there will be somebody watching you on video amazed by how tolerant you were.
Screen: B+. Only dinged for touchscreen responsiveness and general slowness, which is probably more software than hardware. Text looks excellent, but needs more use to really reach a verdict. Backlight is a very nice feature, and pretty well implemented in software. Low battery use of eInk screen, like all Kindles, should be excellent, though how the backlight affects it remains to be seen. Highlighting text seems to work well (it's a disaster in the Kindle app on Microsoft Surface RT, when I read with it I actually do highlighting separately on my iPhone, which is completely stupid), though of course no color. Haven't tried the dictionary, but looks like it should work pretty well.
General form factor: A. I suspect the Kindle app running on a newer retina iPad would be just as good or better than the Paperwhite in many ways, but this is small, light, and much cheaper. (Not sure how it compares to Android tablets, though.) But this is something that will be great on the nightstand, in a plane, travelling, etc. Also stupidly easy to lose on a plane, but I guess an iPad is too. :)
So...gonna keep this one, I think, unlike my previous Kindle. They've obviously come a long way and hopefully this one has room to grow with some software updates.