What are some alternatives?
When comparing Pianu and Synthesia, you can also consider the following products
Flowkey - The easiest way to learn piano with your iPhone or iPad
Simply Piano - Fast and fun way to learn piano
Playground Sessions - Online piano learning game.
Skoove - Babbel for learning the piano
Musicope - The Piano Game. Contribute to oldrich-s/musicope development by creating an account on GitHub.
Piano From Above - Piano From Above is a program that turns learning the piano into a videogame.