As others have noted, it has a high learning curve; but if you're willing to put in the effort, lilypond produces some of the best looking sheet music out there.
Produces beautiful scores. Powerful and comprehensive. Unlike the commercial WYSIWYG packages, software updates won't suddenly move glyphs around. Because you've always got your source code, the program can't irretrievably crash your score. It does take some determination to learn its complexities.
Profesional, elegante, poderoso. Si sos músico y amante de la consola, es para vos.
Very powerful, but its very concept (WYSIWYM music typesetting) implies a steep learning curve that would be eased a lot through more comprehensive templates
This is my number one software as a composer.
Amazing, high-quality output. Infinitely configureable.
Produces great looking sheet-music
Combine with TuxGuitar for awesome scores ;)