it's a "do as you see me do" sort of "method" - I did it for like 4 years when I was 5 or 6 years old. You don't learn about theory and note reading (although, interestingly, you do have the notes in front of you) - but you sit with your parent, and they play it, and you repeat it by doing what you saw them do. So in essence, it's pure ear training. I can see it's worth as a very beginner introduction, to get the fingers working and such for small kids, but I don't think it's something anybody would want to stick with. I don't recall much from those days.. I quit piano out of childish rebellion unfortunately.. took me 20 years to find my way back

Also, I believe it was originally intended for mono-phonic instruments (i.e. Violin) but I could be mistaken.
-Paul