UPS **finally** delivered my Clara HD last night. After a few hours of reading here are my first impressions moving from a 7+ year old Kindle Paperwhite 1 that was ordered on announcement day.
I like that it's slightly smaller and lighter than my Kindle, with the same screen size. It fits in my pockets much better. That said, I found it slightly harder to hold than the Kindle, it just didn't feel as well balanced in my (proportional to my 1.92m/6'4" frame) hand. The smaller bezel did mean that I had to hold my hand in a slightly more cramped position. I suspect I'll get used to this.
The color changing lighting is VERY VERY nice. It's the only reason I upgraded, and I'm very happy with it. Mine has a slight bright spot near the bottom page numbers at night from one of the warm LEDs, but not too bad, so I think I'll keep it (my PW1 had some uneven shadowing at the bottom, which I got used to also). Putting it side-by-side with my Kindle in a dark room is shockingly different -- HUGE improvement.
I like swipe to adjust brightness (though it's a bit touchy), but REALLY MISS pinch to adjust font size. I didn't think I'd care, but it turns out that I really do.
I need to look around the dev forums, hopefully somebody has solved this.
Speaking of pinch to adjust font size, I probably wouldn't care as much if it would keep a more consistent size when switching between books/articles. On the Kindle I seldom had to change it, but it feels like I have adjust the Kobo on every book/article.
Page turns are a bit less responsive than the Kindle, I feel like I have to tap a bit longer to get them to register. I'll probably adapt.
I didn't think I'd care about higher resolution so much, and toying with the PW4 in the stores side by side w/ the PW1 had me convinced that I didn't care about the resolution, but I have to say that it was nicer than I realized and immediately obvious. Not sure why I didn't get that impression when playing with the PW4. I found myself reading with a smaller font than I did on the Kindle.
On my Kindle, I had the official leather cover. The original ones (PW1-3) were way nicer than the current gen cover (PW4), and is very nice. Unfortunately Kobo has far fewer cover choices, and the one I bought (Finite, slim sleep cover), while nice, isn't quite as soft and has sharper edges. Hopefully it'll break in a bit, if not I'll keep looking for something nicer.
Overdrive integration is nice, though the built in search is a bit clunky given that it can only search one library. Easier to find books on my phone and let them auto-sync down. I've yet to get a second library to work, even though I'm logged in to my Overdrive account. I might try setting it up again, but it's clunky, especially entering my 20+ character randomly generated password with lots of symbols, some of which took a bit to figure out... (hint, long press on a similar symbol to find the ones not on the keyboard)... e.g.
Pocket integration is really cool -- loving that.
News support was nicer on the Kindle. I liked how it presented the stories in a newspaper type of format w/ boxes. The navigation isn't quite as good on the Kobo, but good enough.
I've not yet put any custom fonts on it, not sure if I will.
I'm sure I'll discover more as time goes on, but so far I'm happy with it.