In the end,a masterpiece about identity,perhaps of a character,perhaps of movies as an art form (what should we watch? What does it mean to be a watcher in a movie? Should it be a passive,artificial purpose?) and perhaps even of America in the first half of the XXth century.
While some like to negate its merits by it being "overrated",said people just can't surpass the veil of simplicity the movie creates for itself,so the notion of mistery is reinforced.