Absolutely my favorite sci fi movie over the years. Very believable and well acted. It spooked me a bit as a child however I loved to see it whenever it came around the local channel showings.
My favorite line among many was when Klattu told the boy that war had been done away with on his planet... Billy Grey, whose father had died in the war responded... "no wars... that's a great idea!"
In the context of its day it built upon the distrust which grew out of the cold war and supports he Brookings Institute concern of how visitors from space would be feared and dealt with.
Finally, the black and white production just adds to overall feel of the film...something I much appreciate.