Imagine lord of the rings, replace different lands with different worlds, add more characters, more scientific theory and the main characters as young children. That is a very simple way of describing HDM. The books follow the journeys of Lyra and Will, 2 young teenagers as they embark, not just on a comming of age but also on a journey to save existence itself.
The cast of characters is so variable and bizarre that when the films are eventually made, the sfx department is going to be pushed into new frontiers. Deamons, Armoured Bears, Spectres, Ghasts, Ghosts, Witches, Harpes and little people inhabit worlds of the living, dead and all inbetween.
The place this book excels is it's attack on the church. Fully justified and explained throughout the book, Pullman stalks and slash's the church with satire (for instance having an angel couple who are gay lovers)and outright rants (The churches horrific treatment towards 'sins').
Anyone will love this book. The only people who attack it are the people it attacks itself. Ignore them - this book rules.