This is a brilliant film about a man battling his own insanity done in stunning melodramatic fashion by Rainer Werner Fassbinder who was the master of documenting German history and putting on full display the decadence and arrogance of each time period he portrayed on the screen. Also the cinematography by Michael Ballhaus is simply amazing with such vibrant colors making this feel like a Sirk film on acid.
The best thing about the film is the way Fassbinder uses the production design to "imprison" the central character; often shooting him fragmented, through doorways, windows, or as a figure behind panes of glass. It helps to give the impression of a character trapped by the parameters of a situation, or idea, both political and psychological.
Another thing Fassbinder won't let us kid ourselves about: the liberating possibilities of multiplicity. The sense of the Nabokovian source does not survive the translation to cinema -- we know with a certainty that Hermann's double is no double; the ambiguity of the unreliable narrator becomes more obvious and less interesting here. What we are left with is an insistence upon essences, and that they will always out.