If you're a professional composer, arranger, or copyist--or anyone else looking for the absolute best in notation software--Sibelius is one of the two programs you should be kicking the tires of (the other being rival MakeMusic's Finale). Sibelius offers comprehensive notation features including the advanced notation required by modern composers, plus music OCR via the bundled PhotoScore Lite, .wav to MIDI conversion with AudioScore Lite, and a complete collection of orchestral sounds.
Sibelius is easier for beginners to get up to speed with than Finale, though it's still easy to get lost in the labyrinth of features and can't match Notion in the interface department. Pianists and guitarists will appreciate the ability to enter notes via the onscreen keyboard and fretboard.
Note: This link takes you to the vendor's site, where you can download the software. As of this writing, the current demo is v6.1 and does not allow you to save your work; vendor Avid has already released a full version of Sibelius 6.2.
--Jon L. Jacobi