As a nice bonus, Lilypond is also usable via a graphical interface, Denemo [1]. I've never used Sibelius or Finale so I can't comment on how they compare.

What I really wish were available, however, is the IMSLP catalog [2] in digital form (not PDF scans). Is there an app out there to OCR sheet music?

[1] http://www.denemo.org/index.php/Main_Page

[2] http://imslp.org/wiki/Main_Page