Honestly, this is my opinion, I've used many other apps like memrise and duolingo which have systems like how you mentioned, and they never really got me hooked, however strangely enough it's lingvist with its simple method of learning that for whatever reason has kept me interested in it. However one suggestion I do have, is if at all possible make the statistics part of the app more comprehensive, not that it isn't already awesome, but if you could make it more awesome, that would be, well, awesome!