I read books for the characters, and Haddon did a beautiful job of developing the main character of this book. Told from the first person perspective, the narrative voice is charming and credible. This is a child (teenager?) who actually sounds his age rather than sounding like a miniature adult.
The majority of the book is actually tangential information (much of it mathematical) because the point of the book is to communicate how the autistic brain works, how easily it's overwhelmed, how easily it's distracted and how the person copes. But rather than listing it dryly as I just did, you experience the overwhelming and the distractions and the coping strategies with the main character. You also get some insight into how his parents have (or haven't) coped with the limitations imposed by his autism (no hugging, for example) - though this is all filtered through the lens of the main character.
The plot had a little more to it than I was expecting and the story was fine, but the story isn't the point. It's a vehicle to showcase the character.