I'm not usually one to go for old black and white films, but I absolutely loved this, enough to make me consider watching more older movies. The entire film takes place in one room, with the same 12 characters, with no real action other than talking or walking around. Somehow, though, it stays fascinating throughout thanks to its brilliant deconstruction of human interaction. Admittedly, it's predictable, but the intrigue here is more about how you get to the end than about what the end is (though some sort of twist could have been amazing). It also falls prey to some 1950s overacting and theatrics near the end for some characters, but the general theory behind their actions is nonetheless sound.
Basically, if there's a movie that will convert you to slow, old, dialogue-heavy oldies, this is probably it.
In a sentence:
A brilliant deconstruction of human interaction coupled with great cinematography and acting that nonetheless features a bit of 1950s cheese near the end.