LilyPond's text files aren't particularly difficult to
understand or produce, for instance
title = "A scale in LilyPond"
c d e f g a b c
produces a C scale in the treble clef. However, the commands and syntax can also be as complex as music itself. Music entry is tedious, even with a mouse, and with LilyPond it can be downright drudgery. That said, the results I produced in PDF form were nothing short of splendid--as good or better than any program I've ever tried. Also, you can reap the 1500 fruits of other people's efforts by downloading LiliyPond files from Mutopia.
LilyPond is part of the GNU project and as such, represents a fair amount of altruism on the programming and music community's part. When some of the other music programs out there start exporting LilyPond files--or when someone sticks a visual editor on top of it (there is at least one at http://www.denemo.org/ in development)--it's going to be something else. As it stands, it's something anyone who wants to produce good-looking sheet music should at the very least familiarize themselves with.
--Jon L. Jacobi