I know that in the USA, this movie is usually underestimated, maybe because "you-Americans" ( ) know only a version cutted and deeply shorter than the European version.
I consider it one of the greatest movies of all times. You can't absolutely do any comparison beetween "Once upon a time in America" and "The Godfather", or any other gangster movie. Sergio Leone doesn't talk about gangsters, he talks about the meaning, the significance and the importance of *time* for human beings. In "Once upon a time in America" there is not a present time: all the motion picture is structered by flash-backs and flash-forwards. The movie has an intensity so high, and a philosophical meaning so deep, that we can compare it to "A la recherche du temps perdu" by Marcel Proust.
Ennio Morricone is a genius, all his soundtracks are amazing (I also went to a concert of his, in the Verona Arena! )...
I'm sorry that I can express with the correct words all my admiration for this movie.... but trust me, European critics and audience are in harmony: it's a masterpiece.