What are some alternatives?
When comparing Pianu and Piano Notes Fun, you can also consider the following products
Synthesia - A fun way to learn how to play the piano.
Yousician - An interactive guitar-learning app.
Simply Piano - Fast and fun way to learn piano
Flowkey - The easiest way to learn piano with your iPhone or iPad
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.
Playground Sessions - Online piano learning game.