I only knew 2 things about this movie going into it. 1) It's considered "the worst of the best" (i.e. the worst film in one of the best trilogies) and 2) Sophia Coppola was a big reason for that. Well... I wasn't lied to. Michael (Al Pacino) is trying to go legit, though they "pull [him] back in." Particularly, his nephew Vincent (Andy Garcia), a hot-head, is ruining things for him. So while he's dealing with business deals that involve the Vatican, Michael is also trying to deal with his nephew's affairs, including a more literal one with his daughter (and Vincent's own cousin), Mary (Sophia Coppola).
So... let's start with the incest. Tis a bit strange only 1 person in this movie finds the relationship at least moderately strange. But even then, I'm not entirely certain it's because the two are related and more that one is Michael's daughter and the other a wannabe-gangster. There's really only one line in the movie that makes note of the fact they're related. But anyway, that's not really the problem with Sophia Coppola--at least in my opinion.
So while on the subject of what is wrong, let's look at the acting. Yeah, she's pretty rough in it. There are a couple others who could use some help, too, but she's the worst. Pacino is fine, though--just as with the previous film--I couldn't care much about him or his character. I maybe liked his arch a little more in this film than the last one. He seemed more vulnerable and human this time out. Ironically, I liked Andy Garcia the most... and he was essentially playing a classic Pacino-type character.
You could probably easily guess my opinion of this film. The first is most enjoyable due to the character of Vito. The second's best bits are the Vito flashback sequences with De Niro. There is no Vito in this film, flashback or otherwise. If you need it a little more spelled out, I didn't particularly fall for this movie. The best parts are with Pacino acting more human and Andy Garcia acting more like Pacino. I couldn't care less about all the Vatican stuff or any of the politics of the businesses. Like the whole of the film, it just wasn't all that interesting. It was capable and overall well-made, but nothing "great."