I can sightread my own ABC because I make an effort to make it readable; Nigel Gatherer's is just as easy. I wouldn't have a prayer of sightreading John Chambers's or stuff that uses Gonzato's typesetting directives.

Not very different from staff notation - I can easily sightread David Young's 18th century Scottish tunebooks, or Bartok's field notes, but no way could I play off one of Janacek's manuscripts or one of Beethoven's umpteen-times-revised efforts.