koreader is wildly unintuitive (just try to look at battery statistics, and marvel that it's in a submenu of a submenu with only 2 items) -- but it is very powerful. once I got used to it's quirks I can't go back. Things i use all the time on my phone-sized ereader :

1. calibre sync built in 2. customised estimated time remaining/progress bar 3. custom fonts and margin support 4. hardcover.app sync