Definitely the weakest book of the trilogy, which is disappointing since it's the last one. The first book was really the strongest in terms of a story, whereas the other two, and particularly the last book, felt bogged down by the introduction of too many new characters, various complications and twists in the plot with no real climax, as well as having to explain all of Pullman's very interesting, but rather complex and somewhat confusing, ideas.
However, I really enjoyed the last few chapters in which things were explained and wrapped up, all the ideas brought together to form a whole, and you can really see the full spectrum of the complex universe that Pullman created. He says a lot of things about religion and criticizes it rather harshly, but the ending also hints at a kind of altruistic philosophy of life that emphasizes the present and is certainly not devoid of the spiritual. Despite the long and occasionally tedious story, I thought this series had a very powerful, heart-wrenching and satisfying ending.