What are some alternatives?
When comparing Piano Notes Fun and Simply Piano, you can also consider the following products
Yousician - An interactive guitar-learning app.
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.
Pianu - Learn to play the piano in your browser.
Jameasy for Violin - Play rhythm game with a real violin. you can play with background music and real-time feedback
Touch Pianist - Play piano like a pro without playing piano.