Godfather III should have been excellent, and has some brilliant bits in it, but....something about it just doesn't seem right. Like a lot of others here, I feel the storyline is a bit weak, and i found the bits with Michael and Kay "getting back together" particularly nauseating.
On the plus side, I like the way they brought back some minor characters from GFI - such as Enzo - who makes the cake at Michael's party at the start of the film, and Calo.
The part at the end where the Corleone Family strikes back at its enemies was great, in keeping with the first two films, and Al Pacino's acting when Mary gets shot is brilliant.