What are some of the problems you have with the interface? I've certainly got some issues with it, but they're mostly at at the small niggle level, rather than completely off-putting level.
(Should also mention that the interface I am discussing is the one with which I’m familiar: their current iPhone app as-of 2012–2013.)
There is also no capability to discuss only up to a particular portion of a book or any easy way to read books together etc.
The product has a lot of potential, but there are some fundamental usability issues holding it back imo.