[The story of Billion Dollar Brain is apparently as confusing to the central character, Harry Palmer,] as it is to us. He drifts through the whole thing looking at times bewildered, at times merely bored, and quite honestly I wasn't surprised….

Particularly sad is that this hotch-potch is directed by Ken Russell…. He has always embraced the complex structure in his work, and he is one of the few genuine stylists working in the British cinema. Nevertheless, I would rather have seen him tackling a more compatible subject…. [Harry Palmer] is the nearest thing to a Le Carre hero the comic-strip has yet thrown up. Now he is required to enter Bondland and he just can't cope. Perhaps the whole thing is just another symptom of the imminent demise of the cycle.

Richard Davis, "'The Billion Dollar Brain'" (© copyright Richard Davis 1968; reprinted with permission), in Films and Filming...