We get another monster that seeks out energy, but I like this movie. The action moves along briskly, and the story is interesting. The way they handle the special effects is very smart. There is a child-endangerment scene fairly early on that figures as an important part of the plot. I like the 2 short scenes in the church. In the first it's just treated as a normal location where the scientist finds the boy's companion to enlist his help in discovering what happened, and in the second it's used as a place of shelter out of the monster's path.
GoogleVideo has this gem online:
StompTokyo calls it "fake Quatermass" and says,
Once again, Hammer is dealing with a next-to-nothing budget, and for the most part, does it well - one of the advantages of having your production manager write the script. The beastie is not seen until the final third of the picture, and then the blob effects are done extremely well
ImagesJournal calls it "required viewing for all sci-fans" and says,
X The Unknown comes from Hammer's mid-'50s period, when the studio still frequently made black-and-white movies and before they had discovered the path to greatest profits was paved with sex and gore. It's an exceedingly intelligent and thoughtful film, easily superior to subsequent movies about amorphous blobs
DVDTalk has a review.
12/27/2009: SFSignal offers it as their Sunday Cinema post today.