Like all tools, you have to know how to use this one to best advantage. Spaced-repetition works amazingly well. Ive set up customized kanji and vocab decks and my reading ability has improved immensely. I would like the option of adding custom entries for vocabulary, but the existing list is huge so its only a minor request.
The "kanji draw" feature helps to reinforce the kanji in your memory (as long as you try to draw the kanji yourself on paper or the iPad). Its overly critical with the way I draw the kanji on my iPad mini, but that may be because its more difficult without a stylus. I didnt find "kana draw" as useful because the kana was squished to one side and didnt take up the whole square of space nicely.
The grammar portion is still in "beta" more or less. The example sentences use vocabulary far above my JLPT level and I spend more time looking up the vocabulary than working on the grammar.
For those looking for specific features, it does NOT have
1) audio samples for vocabulary
2) kanji lookup from your hand-drawn kanji
3) pull a specific kanji flashcard out from ALL the kanji it has (although you can kind of do this by creating a custom deck)
But Im not docking any stars. Kanjibox is first and foremost a kanji memory tool and it does its job beautifully.
UPDATE:
With the latest version, the Vocabulary and Kanji are inter-referenced. I love this feature! When Im having a hard time remembering a compound word, I can now easily see the meanings of its component kanji and see how to draw them. And then go right back to the rest of the vocabulary drill.