I have refrained from commenting on this thread, but perhaps I now I will share my experiences. As a Sibelius user, I have spent years off and on trying to come up with performances of the string quartets I was writing using the Special Edition. I did not consider my audio files to my liking. I did not see the point of spending money on Solo Strings I when I did not like what I had created using the SE and some of the files were based on SE quality samples. I came into some unexpected money and with reservations I purchased SS I Full.
The first thing I noticed was the all my previous Sib files were sounding better. While watching VE, I noticed that the soundset was choosing a greater number of samples because it had more from which to choose. Combined with VE (which uses your VI, I believe) and maxing out the humanizing features in VE, I was able to produce performances that it not make me cringe. Add to that MIR and I not need to jump for the "Next" button on my iPod when my pieces came up in public.
Should you buy SS I? That would be, in my humble opinion, whether you can afford it. I do not believe that my current pieces have any more commerical value that my SE versions. A person wanting to hear a string quartet will prefer the real thing. While I do like what I am able to do now, none of the results have made any financial gain. The string players liking my music has not increased just because I used SS and not SE. Some of my orchestral music has been performed by real orchestras. In every case, the conductors were uninterested in my mp3s and have said that they will "hear" my composition by looking at the score.
My SS I string quartets can be heard here, but I warn you, I am no Beethoven!
www.markthomas.ca/virtual-string-quartets