It's a great reader app, but as a reader of Japanese light novels it's sadly not fit for my personal purposes - last I checked vertical text is only possible in a really, _really_ hacky way (using a font that is turned sideways), and dictionary support for Japanese (or, say, Finnish with its complex lemmatization) is lacking behind browser extensions such as Yomitan, or mobile apps like Hoshi Reader that have Yomitan dictionary support.

I don't think KOReader handles fixed-size epubs terribly well too, and most of my manga (usually bought on Rakuten Kobo) comes in that format.

For reading English or native language books on non-Android e-readers it's a godsend.