Sometimes these low-budget science fiction features are amusing for unintended reasons, but this one is enjoyable because it has an interesting story and because it is well-crafted for such a low-budget movie. Since its effectiveness comes mostly from the story and from the solid acting, even with a bigger budget it might not have been significantly better.
The menace of "X the Unknown" is based on a far-fetched but interesting concept, and the story gets good mileage out of the premise, without pushing it too far. There are a couple of plot holes, but not so bad that they pull it down, and in general the story has the kind of internal logic that holds it together as long as you grant the premise for the sake of a good story.
The cast does surprisingly well. The characters are not the kind of roles that are particularly difficult to play, but they are the kind of roles that are easy to overplay, and that often are overplayed. Here the actors make their characters lifelike without trying to grab the attention, and it works pretty well.
Dean Jagger has good presence as Professor Royston, yet he avoids the kinds of stereotypes that sometimes make such characters annoying. Some of his little habits and mannerisms help to make the professor more human. Leo McKern also turns in a good performance. Edward Chapman's character is quite one-dimensional, yet he does a creditable job with it. It's also rather fun to see a very young-looking Anthony Newley in a small but entertaining role.
All of the cast members do a good job with a script that is plain but workmanlike. The special effects are rudimentary in technique, but they are good enough, and it's certainly preferable to have plain-looking special effects and a good story than to have flashy visuals and a dreary, pointless story. As a whole, "X the Unknown" delivers good science fiction without frills or padding, and it's easily among the better of the 1950s low-budget sci-fi movies.
