I just watched the classic, I Married a Monster from Outer Space (1958), and actually thought it was pretty good for a 1950's sci-fi b-movie. I'm even more impressed after reading Bill Warren's essay on the film in his book Keep Watching the Skies! and finding out it was shot in only eight days. That's impressive. The plot is hopelessly hokey -- alien invaders from the Andromeda Galaxy take on human form in order to mate with Earth women -- but it's played with enough of a straight face that it doesn't lapse into self-parody. In an interesting twist, American firepower proves useless against the alien invaders. Instead they are ultimately done in by Rin-tin-tin's kin.
This and similar movies, like Invasion of the Body Snatchers (1956), are usually seem as expressing the Cold War paranoia of Communist infiltration, and not without reason. The first alien body snatcher stories do seem to have appeared around WWI at a time when the popular press was whipping up fears of German fifth columnists.
However, another perspective is to view the film in psychological terms as a dramatization of Capgras delusion which results in people believing that their spouse, friends, or close family members have been replaced by an identical-looking impostor. This type of story was a mainstay of the works of Philip K. Dick, even in his early short stories such as "Colony" which was adapted for the excellent 1950's radio show X Minus One. (Download mp3 13MB)
The most surprising thing was watching a scene in which one of the disguised aliens complains about the human urges he's having and realizing that this movie was the inspiration for the Star Trek episode "By Any Other Name" written by D.C. Fontana and Jerome Bixby, in which alien invaders from the Andromeda Galaxy take on human form and begin to experience human urges. Who would have thought?