I just got back from watching the movie. I have read the book (twice, now), and the movie pretty much follows exactly what is on the page. Certain scenes are almost re-creations of the panels in the comic.
Overall, I enjoyed it, though it was dumbed down a bit for Hollywood. Dr. Manhattan and Rorschach were top notch. Dr. Manhattan is a great character, because he totally deconstructs the Superman mythology. If a person had god-like powers over matter, space, and time, why would he give a shit about saving people from burning buildings? He doesn't even care about wearing clothes.
Question for you who haven't read the comic: did you guess who the villain was before he was revealed? I've read reviews that said it was telegraphed.