I agree with lots of what's been said in this thread. I have worked with children with Autism and Aspergers for several years so, obviously, I was interested to read this book.
I agree that Mark Haddon isn't necessarily the greatest writer in the world but, in my opinion, he managed to create a pretty accurate insight into the world through the eyes of someone with this syndrome..
As far as it being an adult book, I think that it kind of bridges the gap between what is a childrens book and what is an adults.
I got my son to read it when he was 10 and he didn't really enjoy it that much. Now he is 12 and has just read it again and he got a lot more from it. He was more ready to take on board what was going on and embraced the difference in thought processes that was being highlighted.
It isn't a literary masterpiece but what it has done is bring some of the issues around Autism/Aspergers to the fore and that can only be a positive thing, right? That's one of the roles that literature plays in our lives and I applaud Mark Haddon for it.