A compelling ride downhill.
The opening is simply spectacular. A flying saucer approaches Earth and lands. The optical effects are wonderful - the saucer even has the necessary shadow. It helps that this movie contains fantastic music throughout. The anticipation of just what that huge machine contains and why it has come to Earth creates tension that cannot be cut through with a blowtorch either.
Things gradually fall apart though. There's a shot of a crowd running away that has been comically sped-up. At the hospital, no-one thinks to photograph the alien's face. In all the public hysteria surrounding the alien humanoid's escape, no-one at the boarding house suspects that this mysterious newcomer just might be the wanted fugitive. When the world's electricity is switched off, this somehow stops cars. (all right, I don't know how cars work)
It's all rather well summed up by that army guy who pricelessly declares "I wish I could be more specific but, as you know, all communications are out. Telephone, radio, cable -- everything." ... just as someone hands him a communication!
As soon as he has served his purpose to the plot, the existence of Bobby the kid is immediately forgotten by everyone, apparently including his own mother.
It's a difficult context for the drama to overcome, which makes it all the more impressive that it does so. All four leads play this very well. As oppressive forces close in on Carpenter, the escalation of double-sided paranoia is conveyed very well indeed.
Since this film was made, the world has moved on a bit. The Russians are no longer considered a threat, scientists have made too many dud predictions to remain the voice of reason, and far fewer people wear hats now. Yet the characters and simple unfolding mystery here still overcome all this.
Well, right up until the film's frankly idiotic ending, which no amount of sincere acting or strong direction can save.
Klaatu: "For our policemen, we created a race of robots. Their function is to patrol the planets -- in space ships like this one -- and preserve the peace. In matters of aggression we have given them absolute power over us."
You… did… WHAT?
Mindless smug self-loathing reason-deficient Vista™-headed brain-donors.
Oh, so I suppose that now you just have to travel the universe threatening to genocide entire species in case they might disagree with what your precious police say. I guess you never found out about power's corruptive qualities.
Following their own example, I guess therefore the US army was morally justified in shooting him.