Agatha Christie was one of the greatest plotters ever. We agree on that, right? (If the answer is "No", then go read a bunch of Golden Age mysteries and come back; then we can have an intelligent, high-toned discussion where the names "Ellery Queen", "Ngaio Marsh", and "Carter Dickson" get thrown around. I'm looking forward to it!) So the underpinnings of this movie are rock-solid. The twist at the end is both surprising and justified. From a whodunit point of view, the movie is impeccable, as is to be expected.
What always comes as a surprise to me is how much fun the acting is. Charles Laughton and Elsa Lanchester are terrific and appear to be enjoying themselves greatly. The courtroom scenes cause me to roar with laughter at the same time that practically every line of dialogue is essential to the plot. Man, I like this movie.