How bad is it? It's filmed well, not uninteresting and has Paul Naschy.
Should you see it? Yes, but not because it's terrible, but because it has a little of everything.
Paul Naschy wrote and starred in a ton of Spanish films, most involving werewolves. In this one, a mystic raises women from the dead to commit revenge killings. The cinematography's good, the costumes are good, some effects are good... so why does anyone call this a bad movie? Well, there is some overacting, a chicken gets its head cut off (and later a guy has his throat slit), the background music doesn't fit, there's a horned Satan drinking blood from a golden cup, evil twins, grave robbing, voodoo dolls, a scene in a slaughterhouse and one or two plot problems. I think it's actually one of the more enjoyable Naschy vehicles; I mean, compared to, say, "Dracula vs. Frankenstein."