I like it. The screen is just the right size, you can change font size to your liking (as long as it’s not a manga), and dictionary lookup works great most of the time (again, not for manga, because there’s no actual text, it’s all an image). It uses the default iOS dictionaries (J>J and J>E) which are quite good. (Dictionary lookup from within Bookwalker is a disaster on Android, because it seems you get no dictionary choice, just google translate, which I find unbearable.)
My only gripe is that whenever I switch to another app for any reason, when I get back to Bookwalker it takes an annoying second or two to reload.
Edit: To OP, if you also have a stylus, the new handwriting recognition is pretty neat. Details in this thread: