Very good, though the story is very dramatized (and I'm Italian, so I know the story of my country).
It takes a real gangster group, link it with neo-fascist and communist actions, real characters and fictional ones, creepy cospirative figures...
It's very far from the criminal stories seen up to now in our country: the cop-movies was a genre abandoned since the '70s (they flourished in the period portrayed in Romanzo Criminale, mostly as a consequence of the feelings of the people against the political and criminal acts of these years).
But it's a very good movie, with solid story and great musics, one of the best in Italian cinematography of the last years (with "Dopo Mezzanotte / After Midnight" and "Le conseguenze dell'amore")