What are some alternatives?
When comparing Piano Notes Fun and Rocksmith, you can also consider the following products
Yousician - An interactive guitar-learning app.
Clone Hero - Clone Hero is a free rhythm game, which can be played with any 5 or 6 button guitar controller...
Simply Piano - Fast and fun way to learn piano
Frets on Fire - Bonus content.
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.
StepMania - Rhythm game for Windows, Mac, and Linux.