I like you, Jules, but you did say that. it was your first comment, I think:
I don't think many, today at least, would confuse the Godfather for being anything like a "realistic portrayal" or the mafia. I think what it did, at the time of release, was portray a different view of the mafia from what came before (think the Cagney and similar films--Irish vs Italian notwithstanding). Basically: a code of honor in a formalized, structured family system rather than a disorganized rabble of petty thugs and ruthless criminals.
The characters in Godfather were inspired by real life members ("participants") of cosa nostra, but nothing that Puzo wrote was a true reflection of that life, or how such organizations were structured--their day to day life and dealings.
What was since revealed in the early 80s with Henry Hill and Joe Pistone's stories, that were then made into very popular films, was that the earlier portrayal of cosa nostra as a dishonorable, back-stabbing gang of petty thugs and criminals, constantly clawing away from a life of near-poverty and paranoia, was far more accurate than what was in the Godfather.
None of that detracts from the greatness of the Godfather films, when you look at them as epic character studies, and a sort of grand opera of the rise and fall of a powerful family. I always preferred the 2nd, as I enjoy the dynamic between the rise of Vito and the struggle of Michael to keep the family from falling apart, while he does everything to destroy it himself.