The Farrell-Knightley romance never really sparkles. It can't compete for attention with the dynamic conflict between the sadistic Winstone and Farrell. Too often the romantic scenes seem to get in the way of the real story, possibly because Winstone does a remarkable job as the villain, but equally because, the romantic relationship is peripheral to the main plot. It, like the scenes involving his sister, could easily have been excised without any great loss. Again, it's not that Knightley doesn't dress up the film, it's simply that her scenes raise expectations that are never realized. She plays no role in the main plot.
Also, I must say a word about the accents. There were too many instances where between the mumbling and the thick accents, I found it nearly impossible to understand what was being said. I have no doubt about the accuracy of the accents, but my own preference would be for comprehension.
That said, Farrell and company are great at the violence. The fights and beatings are as convincing as anyone could wish for. When he slams someone's head into a glass on a table, it plays realistically; when he punches a woman in the face, she looks punched. Monahan likes to have the violence lash out from a kind of menacing calm that is especially nerve wracking. He manages to create so much tension that there are times when he doesn't even have to actually show the violent act. He can leave it to the viewer to imagine he's seen it.
The DVD includes "The Making of London Boulevard" as a special feature. This includes interviews with almost all the major actors and some commentary from Monahan about what he was trying to do. He talks about why he wanted to do a gangster movie that wasn't a gangster movie and a romance than wasn't a romance. He talks about why he wanted to set it in London, and he talks about movies that had some influence on his style; one specifically, Blow-Up is a particularly interesting suggestion because of all the images of photography in the film.