Universal's third Frankenstein movie is the last with Boris Karloff. Director Rowland V. Lee does a solid job, supposedly reworking the script as he shot the film to give more emphasis to Bela Lugosi's Ygor. The movie has the look of German expressionism, and it's effective. The novelty of the monster has worn off, but this is still a decent picture, arguably the last time the Frankenstein story is played mostly straight by Universal.
The absence of James Whale reduces the amount of obvious queer subtext, but the acting lends a definite feel of camp to the proceedings. Basil Rathbone as the titular son, Lionel Atwill as a one-armed police inspector, and Bela Lugosi as Ygor all overact outrageously ... Lugosi, who is used to such things, comes off best, and some consider this his finest performance. Meanwhile, Boris Karloff once again imbues the monster with pathos, but he isn't as central to the picture (and he has lost the ability to speak that he showed in Bride of Frankenstein). He is the best thing about the movie, avoiding the camp stylings of his co-stars.
Son of Frankenstein goes on too long ... it's more than 20 minutes longer than its predecessors ... it's good compared to what followed, but it is clearly the least of the three Karloff Frankenstein films.