Extraordinary!
This is a mature app with many options and configurations. The interface is simple and well thought-out, so everything behaves like your fingers expect. The graphics are smooth, easy on the eyes, and well executed.
There are essentially two modes: a playback mode, and a record mode, which equate to rhythm skill practice and testing. In the playback mode the computer plays what are presumably algorithmically-generated rhythms, and renders correct musical notation of each rhythm, animated with a time bar that sweeps across the screen during playback. You can listen (and watch) a rhythm as many times as you like, so you can really get a pattern down before testing yourself. To really expand the possibilities you can choose what rhythm elements to include or exclude when the app builds the next rhythm.
When you enter the "test" mode you get three chances to beat your score. As you play, by tapping an area of the screen, small diamonds appear where each note you play starts and ends. Since the computer is sub-millisecond accurate, you will be hard pressed to score 100% on most of the tests, but you get a school-themed scorecard that makes it fun to see what grade you earned for your rhythmic performance. You can also choose what rhythm elements you want to practice (i.e. triplets, eighth notes, etc.), and set it up so you can practice with one or two hands. Good luck getting more than 80% on the two-handed tests!
If you want to improve your skills as a percussionist, or if you want to improve your rhythmic sight reading skills as any kind of musician, you should seriously consider this app.