I love that it's easy to use, has many options available in terms of different languages/textbooks etc., is very user friendly! It also syncs well between my computer as an app (which makes me happy) but sometimes there are issues where some books won't sync properly which can be frustrating if you're trying to read them offline later when your internet connection isn;t working perfectly at home / work like mine does all too often :).
The fact they have so much content means i'm never short…