2003 must be the year of autism awareness in fiction. This book is mega-popular and seems to be the reader's choice on the subject. It seems that fewer people are aware that the Nebula winner for best Sci-Fi novel that year was Elizabeth Moon's "The Speed of Dark.", also featuring first-person from an autistic character.
"The Curious Incident..." is well-written and seemingly insightful even if author Mark Haddon says he knew nothing about autism or Asperger's syndrome at the time. The text is witty, the narrative original and the plot forward moving. I also love how the chapters are numbered only with prime numbers.
One thing I have gathered is that the book you read first (this one or The Speed of Dark) seems to end up being your favorite of the two. This is certainly the case for me. Elizabeth Moon actually did know something about autism when she wrote her book. The biggest difference for me is that Moon's characters are all likable, where Haddon's are more irritating. This probably lends Haddon's book to being the more realistic of the two.
That said, had I never read Moon's awesome book, I might be rating this higher. As it is, whether it's your first excursion into autism in fiction or not, this is definitely worth reading.