This film is an absolute classic for camp. That is why it was an Elvira and MST3000 classic. Everyone knows the story. Scientist keeps his girlfriend's head alive in a lasagna pan in his basement while he cruises town and tries to find her a body by checking out the local chicks. Finally he finds a real hourglass body with a scar-faced chick's head on top. The severed head makes friends with the failed experiment in the closet and the conehead comes out of the closet and rips off the assistant's remaining "good" arm (his other is not right from a scientist's earlier failure), and the whole place burns down.
The movie scared us so much as kids that my friend wouldn't go into his basement for a year after seeing it. As kids we ranked the scariest movies of all time and this one was number four. Only one of those scary movies was really any good (the Original "The Haunting".)
I had to give this movie a seven rating for the tremendous amount of entertainment value it offers. Its eerie effect because of the crappy production and the weird sexual angle when the scientist looks for the bodies (complete with porno sound track) scares the hell out of innocent children, while the ridiculous aspects make it prime material for watching talking and laughing. I could watch this film tonight and enjoy it while I'd rather go to the Dentist than watch "Chicago" again.
Seven is the most I can give it, because its entertainment value is mere luck. The film , as cinema, is a disaster.

This movie is all ultra-lightweight fluff, predictable from beginning to end. As a Don Knotts vehicle, "The Incredible Mr. Limpet" was much better, with Knott's character there not nearly as incompetent or ignorant. His performance there was toned down, with none of his trademark goggle-eyed stare, although that may have something to do with him being replaced for most of the movie by a cartoon fish. Knotts made a living of playing the likable imbecile, much as Bob Denver did. Neither really seemed to be able to break out to other types of roles, assuming they were simply typecast. It was probably because of the slouch, the wild stare and the high-pitched voice. John Ritter, whom Knotts worked with in "Three's Company," was able to transcend his genre, branching out successfully into dramatic roles like "The Dreamer of Oz," but the closest Knotts ever got was a small role in "Pleasantville." Even Leslie Nielsen was a bad fit here, uncomfortably neither straight dramatic actor as he was at the time nor deadpan comedic actor as he later became in "Airplane!" and "Police Squad."
There's also no way the then-43 year-old Knotts could pass for a 35 year-old, as his character insisted he was. It was as ludicrously unbelievable as Tom Hanks at 38 playing the college-age Forrest Gump.
The film was clearly made on a shoestring budget, very much looking like a hastily-filmed TV episode. It's especially evident in the "exterior" scenes of the "town" where Roy goes after he's fired. It's unlikely even a pre-schooler would be fooled by the Mayberry-like soundstage artificiality.
Even viewing this strictly as a children's movie, it's very disappointing. It's not because it lacks action or special effects, although it does. The pace is much too slow, the situations repetitive. How many times can you watch Roy getting onto a bus? A comedy for kids should at least sometimes be madcap, with breakneck gags, otherwise you risk boring them (and any adults in the theater as well). Movies, even kid's movies, have improved quite a bit in the intervening decades. Even many contemporary comedies were better filmed and written. Disney's "The Love Bug," for instance, at least had some interesting race action.