I learned about this a few years ago and was delighted to hear some actual performances by Debussy of his own pieces. I was unimpressed by the quality of the recordings, though (via replaying on a restored mechanism) so it's great to get a MIDI version now!

How did the Welte-Mignon actually work? It seems almost miraculous that the dynamics can be captured on a piano roll and replayed successfully. Not perfectly, as you say, but pretty damn close.