Sorry, kept to the novel, which was great and very thought-provoking. In an elitist and snobby move, I will now suggest the film probably plays up the gore and violence, completely missing the point of the story. Judging from the book's length, I denounce the manga as a horrible failure; no graphic novel could be thick enough to contain it. In short: read the book.
In short, watch the film as it's very good, rather than presuming to know how it is.