How bad is it? It's a poverty row vehicle, with poor script and acting.
Should you see it? Yes. It's a passable time-waster.
In 1943, the likable protagonist becomes a mad scientist and Karloff is replaced by Bela Lugosi. Here, he's injected himself with the spinal fluid of an ape and now needs human spinal fluid to live. The ape is terrible, but Lugosi as an ape-man is even worse.