I used that too for ~6 months some years back and definitely got something out of it, though it was imperfect. There's too much lag in any such MIDI system for it to truly correct your timing and PianoMarvel doesn't even attempt to judge your volume or the duration of notes, if I remember correctly. But it gives you a reasonable progression of pieces from easy to harder and forces you to actually play them. Also some scores were poorly typeset, particularly things like trills. Overall, it helped me improve my ability to read sheet music fluently, which is what I primarily wanted out of it. I learned more with a teacher, of course.
Trills and decoration in general (rolled chords) for instance are another area where anything midi based will fare poorly, because the 1:1 correspondence between note-heads and keys played is lost. We intend to try to reverse engineer such decorations and to show them properly. With trills and such it can be quite hard because it is perfectly possible to write out the same sequence, a trill is essentially a macro that expands to some number of notes (and some trills can be quite complex, for instance the first few bars of Partita 1 by JS Bach will throw you for a loop if you listen to them and then look at the score).
