There were many things missing or different from the book, but what movie adaptation is perfect?... Samuel L. Jackson was wonderful as Romulus, and the story made a good movie. If you enjoy the movie, you should pick up the George Dawes Green book -- same author as The Juror, also adapted into a movie starring Demi Moore and Alec Baldwin.
Also, on the DVD's "Deleted Scenes" section there is a WONDERFULLY choreographed scene with Rom's "moth-seraphs" which is just amazingly beautiful to watch! It's too bad they cut it from the movie, but be sure to take it in when you're going through the Bonus Features.