Because my grounding in Westerns is a little lacking, I'd never seen Once Upon a Time in the West. However, I am very familiar with the Negativland song "Guns", which uses several audio clips from the movie (along with part of an episode of "Adventures of Wild Bill Hickok" and a cereal commercial), so parts of it were weirdly familiar to me.
It's a great movie, of course. Sergio Leone must have been one of the most patient directors ever; I can't imagine anyone else lingering so long on almost totally impassive faces. But when you've got Henry Fonda, Jason Robards, and especially Charles Bronson, it just seems natural. I wish I'd seen this on a giant screen, because there were shots where Bronson's eyes would have been sixty feet high.
Incidentally, did you know that the story was by Sergio Leone, Bernardo Bertolucci, and Dario Argento? Isn't that insane?