Well, with some films.. even if you have read the book, seeing the film is still exciting and you just can't wait to see what'll happen the next second. The book was like that to me, the film wasn't. It was easy to predict lines etc. That's not to say the film sucked, I don't think it did, but it just wasn't half as interesting as the book, imo.
The thing is, that what I really liked in the book was basically the problem solving and the numbers and stuff (yeah i used to be a math geek I guess).. The characters aren't exactly super interesting cause well, you don't exactly get to know them a whole lot.. And I don't know how well all that stuff worked in the film.. I don't know in a way it felt to me like they were reading the book while showing us what it might have looked like..