I first heard of WHO? when I saw an ad for it playing second pr third feature at a drive-in in the mid-seventies. As an Elliot Gould fan and a fan of little-known movies in general, I've maintained an interest in it for the last 40 years. Today I finally saw it and quite loved it. It's low budget shows but its photography, direction, and acting all help to bring about a most unusual story. One IMDB user described it as follows: This interesting and underrated movie is not a spy story (well, in some ways it is, but only marginally) nor is it a sci-fi flick or a movie about a robot. What we have here is actually a rather deep philosophical piece on the ultimate unknowability and unprovability of subjective reality.
A flop in its day, later video attempts were made to make it seem very unlike the think-piece that it is. The comment below that its from the man who directed BUGSY and RAIN MAN is also incorrect. In his commentary, Gould points out that it's a different Barry Levinson entirely.