I've been using my SONY PRS-505 since the summer of 2009. I had considered getting an Kindle Wifi because my PRS-505 eInk has always faded in direct sunlight - something I've since learned is a defect only affecting a few of those first-generation eInk screens. Borrowed a coworker's Kindle & really liked the new "Pearl" eInk over my older model, but wasn't very happy with the little four-way rocker switch that the latest Kindle uses to navigate.
I really wanted to stick to the sub-$150 price point, and when the Nook STR was announced I popped down to my local Barnes & Noble and was very impressed. Just about everything is better than my two-year old eReader (page turns, ink contrast, resizing fonts, etc) and I definitely like holding the new Nook better than the new Kindle I borrowed.
As far as UI goes (since the OP asked), I don't really care that much. I am happy that the Nook STR gives me the flexibility to change pages by tapping the screen, or swiping the screen, or pressing the physical buttons, but my requirements are simple: I want nearly zero boot time, to always return to where I was reading when I power up, and page fwd/back should be easy & quick. Easy access to the library of books, search functions, highlighting, note-taking, dictionary lookup are all minor things to me.
I do like the IR touch sensor very much, and typing with the on-screen keyboard actually seems easier than it was with the tiny physical keys on the Kindle.
I side-load everything with Calibre and my 100+ books don't half-fill the supposedly-stingy 250MB of user memory. If I ever anticipate been stranded on a desert island with only my Nook and a solar charger, I guess I'll stick a crowbar in my wallet & buy a sub-$10 multi-gigabyte SD memory card
