The Manster started out as a reasonable 1950s mad scientist movie that, quite possibly could have received a Good rating. Then, about half way through the film it started tanking until it hit the rock bottom of brain-numbing stupidity. Therefore, the challenge is how to rate such a film.
Here are the facts: The acting, for the most part is decent. The mad scientist [Tetsu Nakamura] and his beautiful assistant [Terri Zimmern] are solid actors through and through. Peter Dyneley who plays Larry Stanford, the biological experiment, starts out strong but then gets a bit too melodramatic for my tastes. Part of the problem for Dyneley, however is that they give him the most ridiculous "paper mache" monster head to work with. This head is attached to his shoulder and is laugh-out-loud funny. Even the best of actors couldn't pull this one off. His wife in the movie, Linda Stanford, is played by Dyneley's wife in real life, Jane Hylton. She does a decent job of acting and is decked out like one of Hitchcock's women with impeccable clothing and hair. Yet she, also, has a few moments of melodrama I could have done without.
The biggest problem for me, in addition to the fake monster head, is that the screenplay is utterly ridiculous toward the end. The premise of the film is is good but the resolution of how to cure the Manster is sheer scientific dookey. I don't think even 1950's audiences would have bought it.
Therefore, I have to give it a Bad because even though the beginning of the film shows so much promise, it devolves into a red hot mess of a move that keeps the audience groaning long after it's preposterous images leave the screen.
For more info check out the film's entry in IMDB.