The weakness of biographic movies
I think the need to stick to the biographic story sometimes hurts these type of movies.
I mean, you got a great story here, perfect characters, one of Pacino's best roles, and a long, well crafted buildup, but the last third of the movie doesn't deliver much. There's no sonny vs. lefty showdown or a solution to the lefty\donnie relationship.
Of course the ending has great value in the historic sense, but I think that, if the writers gave themselves more creative freedom, you could have had a much, much better ending. And this movie would've deserved it. I enjoyed it more than The Godfather, it's a bit like a modern godfather-style film for younger folks.
just my 2 cents