This application is absolutely awesome for a quick fix on the go and polished enough to merit the price. If you can though, you should consider an application with emphasis on long term retention like Kanji Gold (a free desktop application) or several websites.
First, I would like to pool more and more kanji (starting with 10 getting to 2000+) together for review. Second, I should rarely be quizzed on super simple kanji I have memorized and more frequently with new kanji I frequently forget. Finally, it should know when to stop!
Each time I get one correct, move it "further" into the future which I cant touch until some time has passed. Each time I get it right, move it twice as far (or 1.5x farther, whatever).
Without a feature like this Kanji Flip is just a toy for passing time. It would be a mistake (or a waste of time) to lean on Kanji Flip for rote memorization.