As you probably already know, this book is notable for being the first-person narrative of the life of an autistic boy: he starts out trying to investigate the murder of a neighbor's dog, and winds up in the middle of a family crisis. The protagonist, Christopher, also inserts into the text a variety of things that interest him: math and logic problems, discussion of space and astronauts, and so on.
The most interesting thing about this book is its unique point-of-view. While Christopher certainly can't be taken to represent everyone with autism, it's still interesting to read his thought processes and learn about how he sees the world. But beyond that, what I admired most about this book was its honest portrayal of how having a child with special needs affects a family. Haddon doesn't sugercoat it; this isn't one of those works that portrays special-needs children as a constant joy to everyone around them. It seems that a lot of reviewers despise Christopher's parents, but I see where the parents are coming from--I can't imagine living with him day in and day out for years on end--and I was pleased to see the author take a more realistic and sympathetic look at the family.
Beyond that, I don't have a whole lot to say about this book. The plot and prose style are both quite simple, but that fits the story and the narrator. The most problematic thing for me was that Christopher shows no empathy for others; not only does he not understand emotions, he doesn't seem to care, and he doesn't seem to be attached to anyone around him. That may be realistic, but it makes for a very cold character. At any rate, this book didn't inspire any particular passion in me, and I'm not entirely sure why it's as popular as it is, but it was a quick read and I found it to be worthwhile.