Since nobody else has written about version 7 of Sibelius and the recent events about the developers and Avid, I feel I should comment about it.
The new in version 7 isn't so much the features, it is the interface. Sibelius has the ribbon now, known from MS Office. The ribbon was implemented strictly: no way to turn it off or customise it. Since the developer had to sign the MS rules of interface design, that means the OS X menu bar doesn't contain any stuff anymore. Basically, in order to bring Sibelius' interface in line with MS Office, the developer has decided to abandon Apple's human interface guidelines for OS X. Well, some users may like that, some don't. It's really a matter of taste. Personally, I find the ribbon better than the (not customisable) tool bar Sibelius had before – you know, the one with big icons for "New document", "Save" (a floppy disk), "Print", etc. etc. etc.. Still: I feel a typical (customisable) OS X toolbar, menu bar and inspector would have been a better solution than the ribbon. Some use this app for engraving music, some use it for composing – many different processes, and each one of us has his individual style to use the app, so customisation would allow to reduce clutter. Still: you have to give the devs credit to have the courage to change something in the UI – compare that with Finale, which is still stuck in a sort of Classic Mac OS UI (ok, not the worst of all possibilities). Other notable new features of Sibelius 7: it runs in 64-bit and it got a new sound library included (which sounds quite ok).
Anyway, here the more important part: in July 2012, Avid (which bought Sibelius in 2006) announced it is "streamlining operations". That means they closed the Sibelius headquarters in the UK and the whole programmer team was laid off. As far as I know, Avid has communicated that Sibelius will be developed by a new team in the Ukraine. I am not sure what this will mean for the software – but usually that is the beginning of the end… so I can well imagine that version 7 is the final act for Sibelius (Jean Sibelius composed 7 symphonies, by the way). One can read at the "Save Sibelius" Facebook-page that now the whole Sibelius team was hired by Steinberg/Yamaha in England – to write a new music notation app?
However, given the uncertain situation I would advise not to invest 600$ in Sibelius 7 right now. If you want to get seriously into music notation (play-in rhythmically with a MIDI-controller, Repitch-Tool, etc.) and composing, get Finale. It much more user-friendly than it was 10 years ago, and at this point, I think it's the better app than Sibelius (I have licenses for both). Then there is the new Notion 4. If Finale has it strengths more at notation, this app certainly has its strengths at composing. Very easy to use interface, and the best sounds you can get for the money (sounds better than Finale or Sibelius, in my opinion). Also, it comes with an iPad app, which is very nice. Notion 3 was a bit buggy sometimes, but I haven't tested version 4 yet, maybe it is a more mature app now. If you want good print quality with little learning curve, go with MuseScore, it's free/open-source and a great, great project. Of course there is other good software for the mac, like "Harmony Assistant" and "NoteAbilityPro". Both are capable and have good, well, even some great features, but regarding print quality I'd still suggest Finale, Sibelius or MuseScore (or LilyPond, if you are a coder). Although for composing these two may also be interesting; you really have to try what fits you best.
The situation with Sibelius is very unfortunate for all of us who depend on music notation apps. In the short-term, you can still use Sibelius of course; also, you can get Finale quite cheaply if switching from Sibelius. And now you can export your projects using the MusicXML format. So, at the moment it's not a huge problem. Still – if Sibelius becomes abandonware, we loose all: it is (or was) good to have a strong competitor for Finale, so the devs were forced to do development. Sibelius was/is a very strong competitor. Many features (ease of use being one of them) in Finale are probably only there thanks to the competition. Anyway let's hope Sibelius doesn't get abandoned; or let's hope the old Sibelius dev team can write an even better new notation program at Steinberg/Yamaha.