This is the most useful study tool for Japanese out there, so look no further! This app has been around for a long time and it's still the best of its kind.
The handwriting recognition is really handy, and I love that it will just show you a stream of vocabulary when you open it up.
The ability to compile your own vocabulary lists and quiz yourself on those words using flashcards is one of the most powerful features. You can also share the lists you make via email.
One thing I'm not happy about is that they removed the ability to search the end of a word or phrase instead of the beginning by flipping the (a...) to (...a) next to the search input field. Before if you wanted to look up 「退化」for example, but only knew the 「化」kanji, you could flip it and search all words ending in 「化」to find it, but now there's no way to do that.