It's unfortunate, bu Joe Hisaishi will never be remembered in the same way the other great film composers, from Max Steiner to John Williams, will be remembered. However, every person that I've talked to that has heard this soundtrack loves it.
What Hisaishi managed to do, more than any other anime composer, is create with his music a world for the film. I haven;t seen the movie yet, but I can really imagine what goes on just by listening to the music. And what further makes the music great is that it stands on its own so well -- it doesn't NEED to be linked to the film. If it was written independantly of anything, it would still be a masterpiece.
What makes this soundtrack so great is its ability to evoke emotion. There's so much variety, and it never gets boring. There's everything from a tribal dance to dream-like music in here. The orchestra is used to its full extent, with very brilliant colorations throughout. There's even a couple of new age-type instruments thrown in, and they blend perfectly. And there are two songs on the soundtrack -- both of which are hauntingly beautiful -- no cheesy Disney songs here (which I'll admit are good in thier own way). They do not detract from the score in any way, and are of the same level of quality as the rest.
If there's anything else to say about the music in Princess Mononoke, it's that when I listen to it, I feel as if I'm listening to real music. There's just something very organic about it, and that the world would be a better place if all music was this good. So get this CD -- I guarentee that no matter what type of music fan you are, if you can even tollerate orchestral music, you will love this.