At first I was wary of subscription based apps because I wasnt guaranteed that it would be a good fit for me or my learning style. Having used this app for over a year now, I would say it has played a key role in my expansion of vocabulary in Japanese. I still recommend this app to all my friends learning Japanese as the expanse of levels and the introductory quiz coordinates a starting point based on what you already know.
The spaced-time recognition matrix means words you readily remember will be featured less often while new words will appear more frequently. In addition, its not just recognition by sight but also being able to pronounce the new word as you enter the syllables into the space provided and translating the meaning of the word.
The only area in which I think the app is lacking is a writing section. Recognition by sight is one thing, but have you truly learned a concept if you can only recall it vocally?
KGR21 about iKnow!