I received my Sibelius 5 yesterday and have had a lot of trouble with it so far. Even installation was a problem until I phoned for help. Then loading in my old scores which were created in Sibelius 4 gave me endless troubles. Hardly a score would have the right sounds, there were all sorts of wrong and missing sounds. Sibelius Sounds Essentials is supposed to take care of everything but that is not the case. My existing VSTs were not available, playback had stutters, and so on. I am in touch with the technical boys at Sibelius and I'm hoping that they will sort everything out. It may be that my computer was not up to the task but I would have thought that Intel Pentium 4 @ 2 Gig and 1 Gig RAM would be adequate - it certainly was with Sibelius 4, which never gave me any trouble.
This is disappointing because the new sounds were the only reason I upgraded, hoping to achieve at last the all-in-one program which might make sequencers obsolete. I don't rule that out yet, if my problems get sorted properly. What I am saying is, if you're thinking of ordering, hang on for a bit until I can give you more information.