It makes political comment on consumerist society, America's foreign policy etc. It makes you consider morality.
Joss Whedon is one of the minds behind it, so I kind of knew it would be awesome.
I have been analysing it over and over for about 24 hours. And while actually in the cinema, I was a nervous wreck - the Cabin's self-consciousness doesn't enable its audience to escape the very compelling fear. But every so often, you guffaw as well.
See it, please. (Though possibly not if of a nervous disposition. If I'd watched this at 15 I'd have pooed my pants. Just saying.)