I have been using this App in my studio since hearing about it from Peter Mack at the 2013 National Conference on Keyboard Pedagogy. I absolutely love it! It counts, it subdivides, and the various voices are simply hilarious (there is Darth Vadar!) I recommend it to all my students that have an iPhone or iPad. It is $0.99.
For some reason, this App is hard to find in the App Store, and there is a similar App called BeatSpeak which is not as good. To avoid getting the wrong App, please visit this link.