Sibelius 5 is used by thousands of music educators and professional musicians world wide, and it's rise in popularity in the United States over the years can be traced directly to the hard work the Sibelius design teams have put in to improving the interface, adding requested features, and expanding their pull on savvy computer users as a whole with the introduction of the Sibelius Scorch plug in which allows music created in Sibelius to be viewed on web sites, complete with the ability to transpose, play, print, and save all without the need for a license for the full Sibelius product. One note of warning, files created for Scorch using Sibelius 5 will require the viewer to download and install the newest version of the Scorch browser plug-in before they will be able to view it. You can avoid this issue by saving your files as Sibelius 4 Scorch files using a plug-in inside Sibelius if needed.
Recently I did a review of the new Sibelius 5 release after having been forcibly converted to Sibelius 3 from Finale over a year ago. The differences between version 3 and version 5 are well worth the upgrade. From 4 to 5 still gives you a decent amount of new stuff, especially the newly upgraded MIDI voices thanks to the Garritan Personal Orchestra sounds that are now included with the basic installation. Plug-ins continue to be improved and expanded upon, with many user created plug-ins available to make common tasks even easier. Another, more in depth Sibelius 5 review covers these additions and changes in more depth.
One can only wonder what the future will bring for the Sibelius franchise. They have partnered with InTheChair to produce a SmartMusic style alternative, and they have been bought out by Avid , makers of high end video editing software. As their market share in the United States continues to grow, we can only hope that the usability and feature set of SIbelius continues on its positive rise.