There are a lot of strings libraries. And of course there are a lot of very very nice demos for each of them as well.
But when it comes to produce a mockup of a famous piece (Bach, Handel, Mozart, Brahms) we always land up at VSL.
All the rest sounds poorly, the company doesn't offer such mockups or at best they sound not really better.
Why this?
It's easy to make a fantastic demo along the samples you have.
It's not so easy to come as close as possible to an original with a library because this way shows the limitations of it.
Recently, there was a post about a piece of bach (http://community.vsl.co.at/forums/t/30109.aspx?PageIndex=2) and about "the good old chamberstrings".
...So, do not doubt about the VSL-Strings until you can listen to a bunch of mockups of another library which sound really better than those from VSL.
Nevertheless, I (we?) admit that there are libraries available today which sound very nice and very musically (more than VSL) with single tones/notes.
But strings do change their tones sometimes and more and more we get an impression over all... which can lead back to VSL.
All the best
Beat