I've only seen two of his films, but they were both amazing. Spirited Away is very down-the-rabbit-hole and intended for a younger audience, as is most of his work, I believe. Princess Mononoke is definitely for a more grown-up audience, since there's quite a bit of fighting and violence, but he tackles themes of nature vs. technology, the grey area between good and evil, and personal redemption. The storylines are clear but complex, and the animation is some of the best I've seen come out of Japan.

The New Yorker did a rare interview with him for the January 17 issue of this year, but it was before my subscription started, and it is sadly not posted online.