Beginning and intermediate piano players will benefit the most.  But advanced players will appreciate the  chord analysis capabilities of the program.

Because it focuses on popular song material found in "Fake Books", FastChords is not for aspiring concert pianists.  It is more for people who want to learn to play piano "by ear" and from music they like, as opposed from formal music lessons and years of practice.  Neither is FastChords a music theory teaching program, or a music notation program, or a MIDI sequencing program.

Midi, however, is supported quite well, you can record and play midi performances.  You can even single step through recorded midi to observe your finger work.

Being able to read music is not necessary to use FastChords.   The program has a Melody Mode that maps music notation to keyboard keys, and vice versa.  All you have to do is to visually copy the notes from the printed music. 

Learning piano by practicing with songs that you always wanted to play is the fastest way to get going with something you'll enjoy the rest of your life.