Being totally new to posting notes on message boards, this may come across all wrong, but how can people interested as you are, not have seen Goodfellas, A Bronx Tale or Mean Streets? They are Classic's
This thing of ours, starts with a passing interest in Godfather, you see the first and bam, your hooked. You rent the second and then look around for other related films & books. Before long DeNiro & Pacino are your favourite actors, you've seen nearly all their films, you even like some that are not Mafia based. Although there not as good as those that are.
If your lucky, you would have read the books before seeing the films, as this truly conveys exactly what the author wanted. This being the reason Godfather 3 is not my favourite film, never should have happened.
To end what is the general feeling regarding Once Upon A Time In America....