I've been writing this review in my head through half the trilogy, but i wanted to actually finish the trilogy before setting any of it down.
The quality of Pullman's writing craft i'll give a B. It was particularly uneven with regard to vividness. In many parts, there was no attention given to the senses at all - no description of sights, sounds, smells. Now, typically, i don't like prose which is bogged down in elaborate descriptions of things. But a few hints here and there, just to tickle the senses, would have been effective - especially given the (literal) otherwordliness of many of the book's settings. In other places the setting descriptions were so elaborate the scene felt bogged down.
Dialogue was good though, and the characterization was (with one exception) superb. I love that the protagonist is an untidy, poorly-behaved, stomping-in-the-mud, neighborhood-warfare-waging, prank-pulling, truth-challenged 12 year old girl.
The exception is Marisa Coulter, a femme fatale who wields charm, seduction, and manipulation to achieve supernatural results. Coulter is the hardest character to read, because one never knows when she is being upfront and when she is lying until she actually acts. I know this is by design, and that element of not knowing would be laudable if it were done via any different means; but it's still unfortunate to see a character play an essential role mainly because everyone who meets her is stunned by how she looks.
Still, i have to give Pullman some points for writing a work of fantasy in which female characters are just as strong and prominent -- if not, on balance, a little more so -- as male characters.
The plotting and storytelling i'll give an A. As a whole the work is superbly conceived and structured. It's set in an elaborate multiverse and the reader finds herself wishing she could take tangents, just to learn more about this or that. Lyra's world, where the story starts, is fascinatingly different from our own. Even the experience of day to day life as a human being is vastly different there, because every person has a companion, a dæmon, who is an extension of their individual being and nature.
In many ways, this is the ultimate "underdog" story. The heroes are figures usually cast as villains: witches, fallen angels (esp. gay ones), dæmons, harpies, users of divination, gypsies ("gyptians" in Lyra's world), African kings, rebels, dissidents... while the villains of the book are figures of authority: various members of the European upper class, bishops and other church functionaries, and upper ranks of angels, including God himself.
Wait, so God is a villain in His Dark Materials? Well, it's more complicated than that. ( spoilerish stuff starts here )
Hmm, not sure how to characterize the last few paragraphs, but since they gel with my own views, i'm going to give it an A. Which means that my overall grade for the trilogy is about an A-.