A few of the things I like about this app are that it teaches all of the hiragana and katakana kanas instead of just a subset like other free apps. It provides a helpful and interesting background on the development of the kanas as well. The quizzes are nicely randomized and all the more helpful because each one includes material from the previous lesson so one can be sure one can identify a character from all characters rather than easily guessing from just the 5 most recent. The app also tests audibly, visually from Japanese to English, and visually from English to Japanese so one learns to fully utilize the kana. I also appreciate the tutorials on the stroke order for each character. Finally, I love that this app is still compatible with old devices like mine.
One additional feature I would love would be quizzes for just the characters that are similar, like ho, ma, and mo; ne, re, and wa; and nu and me. I can guess those when my choices include other characters that don't look similar, but I would really be forced to remember them if I had to pick from similar-looking characters.