Tim Brayton’s review published on Letterboxd:
Part I tells a complete story of one man's fall into corruption, but Part II is what transforms it into tragedy. It is a heavy, grandiose sprawl - we're still in the realm of opera, but if the first film is more like Puccini, depicting with grubby realism the effects of human cruelty and depravity on other humans, this one is Wagnerian, plunging us into a world that has already fallen, watching a man whose fate is preordained from before the moment we first see him, and all we're there to do is watch the inexorable unfolding of doom. I love the bolding and underscoring that go on; Part II feels to me the more intense and unrelenting of the films, even as it spends so much of its time on the historical sequences that are so oddly motivated - even calling them "flashbacks" implies much more of a fixed relationship to the present-day material than we actually get.
But since I've introduced the opera metaphor, let's see how far I can take it before it breaks: we basically have a counter-melody, a story of familial loyalty and immigrant striving to accentuate the A-plot of assimilating into the monoculture and betraying every last family member. The two time periods fit together tonally more than narratively, and other than the fact that it simply feels perfect to me as I'm watching it, I have no evidence that this very odd approach works; but I absolutely wouldn't scrap it.
It's a darker and heavier film, with Gordon Willis leeching out even more color and making the contrasts between light and dark even more jarring, on top of the new plot focus: while Part I focused on the politicking within the Mafia, creating an insular world that functioned, however horribly, on its own terms, Part II is all about the grotesque effects of the Mafia on the outside world, and while I can sort of see how one would walk out of the first film with a sense of romanticism about the honor of the men involved, there's hardly a single scene in the sequel where Michael seems like anything other than a murderous paranoiac.
In short: the original is a film about moral decay, the sequel is a film about moral nihilism. I'm not sure if that's why, for the most part, the original is still probably the better-regarded of the two; I am absolutely sure it's the heart of why I prefer this one. That, and the fact that I would 100% take John Cazale and Talia Shire in this over any of the supporting performances in the first one, and I think Al Pacino is doing much more here, albeit with a smaller palette.
Boy howdy Christ, do I wish that abortion line wasn't in the script, though.