How bad is it? It's ludicrous and, for Karloff fans, a bit sad.
Should you see it? Yes, if you can find a good copy.
Until recently, Boris Karloff's last 4 films were rarely seen and this is probably the one to catch. Scientists find a living rock under a volcano, which happens to be telepathic and also needs the chemicals human bodies produce when frightened in order to live (go figure). There's a dwarf and a giant, a sadistic lesbian, an eastern mystic, Boris Karloff doing almost all his scenes from bed, a go-go dancer getting manhandled by a tentacled rock and a lot of screaming. The Karloff scenes, directed by Jack Hill, are much better than the rest of the film in quality.