A few thoughts: Karloff is mistakenly put to death and gets brought back through the miracles of science, only to find himself thirsty for revenge. With this fairly standard set-up, the movie manages to take a few different turns throughout, particularly in making Karloff less psychotic and more sympathetic. He has some sort of psychic, otherworldly power that makes people die even when he doesn't directly do anything to them, so he never comes across as terribly malicious here. The movie takes a good deal of its running time setting up the story, but sadly never feels particularly committed to giving it a distinct personality. This is a very standard movie with less emphasis on the scientific side of things than on the gangster/revenge portion of the story. Karloff has given far better performances but he is solid here, as should be expected. Still, with a very underwhelming role and little for him to do, he's left with little more than a shambling Frankenstein-type role without the distinctive look. It's all very average.
Who would I recommend it to? If you've made it through the grade-A and grade-B '30s horror classics, it might be worth looking into. But definitely not until you've seen the essentials.