Scorsese's "Birth of a Nation" possible spoilers below (but I doubt it :)
This is a stunning film that pulls no punches. It will not appeal to all.
Is the movie violent? Yes, but not violent for only the sake of violence. To demonstrate my point, let's compare the opening scene in "Gangs" to that of "Saving Private Ryan".
Although overwhelmingly regarded to be an outstanding battle scene, "Ryan" seems to morbidly dwell on a catalogue of deaths and killings. It documents one after the other after the other from an objective point of view. Scorsese's battle scene, on the other hand, is much more subjective. We can feel what it is like to be in the middle of the battle, because everything - bodies, limbs, faces - blurs together. We can hardly recognize who's killing or being killed at times. We are confused, and that's what is so terrifying about hand-to-hand combat like this: You don't know who's coming up behind you as you are killing somebody else.
In Spielberg's film, we know what it's like to see from a third person perspective someone getting killed in a brutal battle. In Scorsese's, we know what it's like to *be* in a brutal battle. This scene in "Gangs" is much like the impressionistic snowball fight scene that opens Abel Gance's "Napoleon", with a notable exception being, of course, that "Gangs" opens with no mere snowball fight.
In short, while Spielberg is intent on documenting facts, Scorsese is more concerned with (as F.W. Murnau would describe it) photographing *thought*. While in the first case we might sometimes wonder if we are just seeing some interesting special effects, the second case leaves no doubt about it: The violence is there because these are violent times.
At the root of this battle is a power struggle between races. The Irish want to take part in the American dream, but the Butcher will not allow them. He considers himself to be a "true" American, and the name of his gang reflects this.
An interesting thing about "Gangs" is that by going back early enough in history, Scorsese is able to document events that we can recognize in other American genre's that appear throughout the history of film.
Most obvious is the gangster film. The power collaboration (as well as struggle) between the Butcher and Tweed is a precursor to that which we see in films like "The Godfather".
Also, as the Irish are coming off the ships, become American citizens, and immediately are recruited to join the Army, we can sense that some of these men would go on to be much like the Irish sergeants which John Ford would feature in most (if not all) of his westerns.
Finally, just as D.W. Griffith's epic depicted the Birth of a Nation - i.e., before the Civil War, many Americans did not consider themselves to be all from *one* nation, but instead from a *state* that was one in a *union* of states - Scorsese's epic depicts the Birth of New York City; the melting pot it would become and pride itself on being.
In one scene, the Butcher comments what a confused and terrible place the country is becoming when he sees a black man dancing an Irish jig. He considers this to be a summary of what is going wrong with the country. We, of course, see the same thing and consider this merging of cultures to be what makes our country so great. We our witnessing the Birth of our Nation.
Finally, as with most movies, leave your watch at home. Allow yourself to be lost in the movie, and you're more likely to enjoy it without fretting over details such as the running length, or who's playing what role. Don't come to see Leonardo. Don't come to see Cameron Diaz. And don't stay away because they *are* there. Come to see the story, and all will be fine.