The Return of Doctor X is not so much a sequel as it is a reboot. In modern film language we sometimes call this a "re-imagining." However, in this case, no imagination was used in the making of this film! Overall it left me with the feeling that The Return of Doctor X was made to cash in on the popularity of the first film. Imagine that happening!?!
This time out the story involves a new Doctor X, with the unusual casting of Humphrey Bogart in the part. He's fine but he cannot light a candle to Lionel Atwill's performance in the first. However, Bogart would get his revenge a few years later with the huge success of The Maltese Falcon (1941) and Casablanca (1942). Faye Wray, who starred in the first, is also sorely missed an no one really takes her place. There is a reporter for comic relief like the first film. But it all feels like it's been done before and done better.
The other disappointment is that the horror elements are gone. Instead of the "synthetic flesh" storyline, we have one about "synthetic blood." It just doesn't work very well and lacks the punch of the first film.
What is left is a decently made crime drama with little originality or emotional impact. What a disappointment.
For more info check out the film's entry in IMDB.