I purchased Sibelius for Windows (version 1) when it first arrived in the late 90s - it was a dream come true because the only way to use the program was to buy a package with an Acorn Computer which was too expensive. I upgraded to version 2, but after a while I really wanted to be able to use VST instruments, however, Sibelius 3 & 4 did not have this feature, so I skipped the upgrades. Now, Sibelius 5 is here and it is great to now have proper VST support without mucking about with MIDI yoke etc.
The notation aspect of Sibelius 5 is first class, scores can be published at an exceptionally high standard, with all manner of complex notation. The GUI is superb and the music output is far better than any other program I have seen. All notation is laid out correctly apart from very complex situations e.g where you have 3/4 parts on one staff (eg Bach fugues) where in this situations there are little guidelines for layout anyway. Sibelius does follow engraving rules very well - these can be tweaked and if Sibelius does get it wrong (which is rare) it's a doodle to correct. It is easy to use and input notes, but to get a full grasp of what the program can do does take time - it can't all be learnt in 2 days, but after a couple of hours a beginner could input in a couple of pages of a Mozart orchestra score without too much difficulty. To be fully up to speed, the best form of input is via the keyboard, as I said, it takes time and you could buy the over-expensive Sibelius keyboard with special coloured keys.
The main Mixer has been redesigned so VST instruments and effects can inserted, the default instrument that is loaded up each time is Sibelius Sounds, a Kontakt 2 Player plug-in. Other VST plugs can be loaded, however, loading Kontakt 2 with the Sibelius Sound plug in I got a strange dialog box about an error (some internal programming line)- pressing the 'Ignore' button got rid of it and the playback of both together did not seem to be affected. The sounds are OK - a bit unrealistic - but it is only 2 GB with the free package with Sibelius. I expect a lot of users will use solutions such as EWQL / GPO / VSL. An important new feature is SoundWorld, a new standard for naming and classifying sounds - using a sound ID for each patch. This does however require Sibelius Software classfying sounds in 3rd party plug-ins - it works OK with my standard sounds with Kontakt 2.
The documentation is superb and Sibelius are well known for this - however for upgrades only a 'what's new' manual is provided and a proper manual is a paid for extra (comes with full versions). This is a bit annoying, espically as I can't find where to order one on their website.
The time that I have worked with it, I haven't come across any other bugs apart from the one just mentioned.
To be blunt, if you are a musician who uses notation, it is a basic, essential, necessary purchase.