Moria says it's "one of the real classics of the science-fiction genre." This movie has been selected for preservation by the National Film Registry. It is on many "classics" lists of sff films. Slant Magazine calls it "The quintessential alien visitation tale of its era". The New York Times describes it as "a tepid entertainment in what is anamolously labeled the science-fiction field." 1000 Misspent Hours praises it, opening with this:
One of the earliest, and most intelligent, modern science-fiction movies, The Day the Earth Stood Still may be a little too far on the cerebral side for some people (if you mainly want to see shit blowing up, you’ve come to the wrong place), but it fully deserves its classic status.
Veoh has this one available, but you have to download their player.
5/13/2008: 1001 Flicks has a review.