I agree with dewdman, and feel that VI is like primal musical samples that one must have. In other words, you can have all kinds of software that is fancy and does things automatically, but if you want just pure musical sounds played by great musicians, in a silent stage that allows you to use it in any acoustic venue you want - VI is the ultimate. I have used many different sample libraries but the reason I keep coming back to VI is because it is so methodically created with regularly organized articulations among all the instruments, and adaptable to anything you might want to do acoustically. It isn't locked into a single venue, or a single "spectacular" sound design effect, but rather it is based totally on what the symphony orchestra does - the ensemble which evolved over centuries and is the ultimate musical expression. It doesn't necessarily give you instant gratification, but neither does a real symphony orchestra - you have to use it and figure out how to do that. And you can actually use VI the way an orchestrator for live orchestras uses them.
This library after all was created by brilliant musicians and the man in charge of it is a great cellist from Viennese orchestras. The company has always placed musical values first and the VI series shows that in a pure, intense form. It has a truly lasting value - it is funny how I still use just as much now as twenty years ago (!) the very first instruments they did - like the trombone, the horn, the C trumpet, the orchestral violins, etc. And the later instruments were just as great. And with the addition of MIR which has the most simple-to-use but beautiful sound venues, it is even more powerful. VI is essentially an endless resource for musical expression.