There have been several threads on this in MMD; check the Archive.
I have done a fair amount of this, mainly for my own research.
The preferred method seems to be to use a notation program (like
Sibelius or Finale; I use the latter) in combination with a MIDI
sequencer (like Digital Performer) with which you can quantize the
beats and make other edits before you import it into notation.

Otherwise, as was said, you end up with a pile of gobbledygook that
requires extensive editing: among other things, it is very difficult
to accurately separate the two hands onto separate staves.

It _is_ wonderful to have the MIDI files to work with, especially
since so many are available on-line.

Best,
Jeff Taylor
jtaylor651@verizon.net.geentroep [delete ".geentroep" to reply]