My only real complaint with KOReader is that the book browsing UI is file and folder based by default, while virtually all other ebook readers support some kind of cover based library view you can sort 20 different ways based on metadata rather than filenames etc. It's not a huge issue, but I find just browsing my collection to find a read a lot more enjoyable on other ebook readers. IIRC there are plugins to add thi…
My experience with battery life seems to be the opposite of yours. I'm using it on a Kobo Clara BW and my battery just lasts and lasts, even with an hour or more a day of reading. I don't have any firm data on it, but I know that right now I'm at 52% battery and it's been at least a week and a half since I plugged it in anywhere.
It still lasts a long time! It's just not as efficient as Nickel on the same hardware.
Personally I prefer Calibre + Nickel over KOReader on Kobo devices (its trivial to convert DRM-free eBooks to formats Nickel will open, Calibre will do it automatically if you plug a Kobo in), although on Kindle devices I would still jailbreak and use KOReader.