I've been using Lilypond for a number of years now and it's a truly remarkable piece of software. The quality of the engraving is extremely high, the amount of customization you can carry out is unrivalled, and it is also incredibly versatile. Out of the box it supports stuff ranging from Guitar and lute tablature[1] and notation specific to percussion[2] to modern chord names[3] and Baroque figured bass [4]. Did you know you can also use it to typeset neumes for Gregorian chant[5] and mensural notation[6]?
Nevertheless I should mention that Lilypond serves a different purpose and is aimed at a different audience when compared to popular scorewriters such as musescore. It is primarily an engraver and should be used to typeset existing music, composing directly into it is always going to be very awkward compared to WYSIWYG editors or even just manuscript paper. The learning curve is also quite large and you must be ready to constantly refer back to its (very well written) documentation. But like any complex tool, acquiring familiarity with it is very rewarding in the long run.