What are some alternatives?
When comparing Piano Notes Fun and Frets on Fire, you can also consider the following products
Yousician - An interactive guitar-learning app.
StepMania - Rhythm game for Windows, Mac, and Linux.
Simply Piano - Fast and fun way to learn piano
Clone Hero - Clone Hero is a free rhythm game, which can be played with any 5 or 6 button guitar controller...
Melodics - Melodics is a desktop app that teaches you to play MIDI keyboards, pad controllers, and drums.
Rocksmith - Rocksmith builds off of the fundamentals of music games like Rock Band and Guitar Hero by offering fundamental lessons in actually playing guitar.