This was a low-budget horror film with very modest pretensions. No one involved believed they were making "high art" and with a small budget and running at only 62 minutes, this is a definite B-picture. And in light of these factors, it's an amazingly effective and enjoyable film.
Boris Karloff plays a mad scientist--this is certainly no great stretch. His research involves trying to switch the mind of one person with another--sort of like the plot that was often used in cartoons or cheesy comedies in the 60s. How exactly this was going to be a GOOD thing certainly wasn't a primary concern for th doctor, though later in the film, greed and an over-active libido push this strange doctor to make this switch with unwilling victims.
So despite a pretty corny plot, why did I like this film? Well, the pacing was excellent but more importantly the film had wonderful dialog and was at times very 'tongue in cheek'. In particular, when Karloff's evil and physically twisted assistant changes bodies with the rich philanthropic newspaper owner, I found myself laughing repeatedly because the writers for the film deliberately injected some levity into the horror plot. You just have to see it to understand and appreciate this.