A few thoughts: One of those rare movies with a single image far more well-known than the movie itself, Mr. Sardonicus fits nicely into William Castle's filmography as a fine example of the charms of his gimmickry. Gleefully cheesy and in poor taste, I can't go so far as to say this is a very good film, but it's an entertaining one regardless. Filled with hilariously cornball torture scenes and close-ups on some very poor prosthetics work, this movie tells an outrageous story that's just goofy enough to be funny and just serious enough to not be a comedy. I enjoy the opening and closing bits featuring Castle himself, moments during which you can tell this was a man who absolutely loved what he did. Were his movies silly? Absolutely, but the way he tried to have his audiences participate in the experience was infectiously charming. As far as his movies go this wouldn't be at the very top of the list, but if you enjoy the look and feel of black and white horror from the '50s and '60s, this is a competently made movie with a cool setting and creative presentation.
Who would I recommend it to? Anyone who likes occasional gimmicks and/or relatively light-hearted horror.