Great! This is one of my favorite movies, one I can watch and enjoy on repeated viewing. All the actors really bought into the tone of the movie, which tells me that the director had a clear vision and was able to communicate it to the cast. Brando is brilliant, as usual, and I find his performance to be highly entertaining in that he spoofed his Godfather persona so effectively. I always enjoy seeing (and hearing--love his voice) Bruno Kirby, and I especially like that he was cast in a movie about the Godfather, given the fact that he had played the young Clemenza in the Godfather II. Penelope Miller also played her part well--I especially liked her character's behavior toward Matthew Broderick's character as she assumed he was going to marry her and be part of the family. Her exasperation with Broderick ("This isn't the Clark I know") was great.
Then there's Matthew Broderick. This is the last movie he made that I truly enjoyed. He is absolutely perfect in this part. His part calls for him to portray his earnest and trusting personality, his loyalty to Brando's character and to the job he agreed to perform, his growing fondness and respect for Brando's character, and his confusion about perhaps having given his respect to such a disreputable character. Broderick carries off all these aspects of his role, yet shows the strength of character to be able to help solve all his problems by the movie's end.
Or, he could have ended up as Rodolfo Lasparri of Palermo, Sicily.