Men in Black III is inventive, funny, and even sentimental (which is new for the franchise). It hangs together well, and is both overtly and slyly funny. It's well acted, and comes at you from all sorts of angles.
It's not nearly as much fun to write -- or read -- a positive movie review as a negative one, so I won't go on at length.
It is, however, fun to see a good movie, so catch this one.
It'll be the most fun hour and forty-six minutes of your day.