I know it does sound sad, and most customer reviews just say "yes, it's great, it's the best book I've ever read" and that sort of thing. In this case, though, I think it probably is.
The intricacies of the plot, the characters and the ideas are incredible and enveloping. The story is fast-paced and multi-faceted. The emotional involvement is second to no other book I have ever read. It is a very sad book in parts, but also very uplifting. It made me believe in love, and life, and all those sorts of things.
You may not be as emotionally malleable as me, and so may not find youself affected so profoundly by the book as I have been, but if you enjoyed Northern Lights and The Subtle Knife, then there is really nothing to do other than to read this one and be astounded. If you haven't read those two titles, then be very aware that this is a trilogy and that it will make much more sense if you read them first.
Above all else this book highlights the dizzy heights that Philip Pullman's story-telling can take you. It is a classic book in a classic trilogy. What more can be said?