I think the LilyPond documentation (http://lilypond.org/doc/v2.18/Documentation/learning/index) is a pretty good way of learning LilyPond. It's linked to from "menu > LilyPond manual" within Hacklily. Maybe I should be more aggressive about highlighting that...
Making community features is a natural next step which might help with finding examples.
LilyPond has a bit of a steep learning curve, but you can absolutely use it professionally. http://www.mutopiaproject.org/ shows what LilyPond can do. Personally, I find that I can be more productive with LilyPond than with Finale, and that the results look better.
I've been using Musescore for a lot of the stuff I used to use Lilypond for, but one of the things I absolutely love about Lilypond is that its easy to write scripts that emit lilypond format, it's killer for repetitive stuff like examples, flash cards, etc.
I was never any good with Finale or Sibelius, so I feel like I'm unqualified to say whether Musescore is worth learning, but have been curious what an actual skilled Finale user thinks about it?
Making community features is a natural next step which might help with finding examples.
LilyPond has a bit of a steep learning curve, but you can absolutely use it professionally. http://www.mutopiaproject.org/ shows what LilyPond can do. Personally, I find that I can be more productive with LilyPond than with Finale, and that the results look better.