I'll amplify what William said about the level of organization. When I was casting around for a comprehensive set of libraries with which to develop my computer aided composition and performance system the VSL libraries stood out head and shoulders above anything else on the market. Not only for the richness and variety of the articulations provided and the virtually complete instrument set but, for the purpose of building any kind of automated system around their use, the consistency of the way the articulations are named and ordered saved me a huge amount of work up front that would have to be done with any other comprehensive library.
Other libraries tend to have many inconsistencies in naming and more inconsistencies with respect to the articulations provided between different instruments in a section such as contrabassi, celli, violas, and violins that one would expect to all have the same set of articulation options. VSL not only does that, but provides virtually the same consistency across libraries such as the various string groups. You get the same consistency across the winds and the brass. That consistency in VSL makes it possible to swap instruments into a mockup and get surprisingly close to the original design but with an alternate orchestration.
On the other hand, there are times when you want a particular sound or performance style and it's simply not available with a given library. Then you go with the resource that best suits your purpose, even if it's not as well constructed as this one.