I finally got round to watching this mad beast of a movie last night, and to be honest I'm still digesting it. But as anyone else who has seen it will undoubtedly attest, I can't really talk about the film at any length (which is good seeing as time and brain cells are both in short supply again today) without running the risk of completely giving the game away. So if you haven't seen it yet I'd say just go in cold. I'll just briefly mention a few things I liked, albeit in very broad stroke sort of fashion so we can keep this sans spoilers.
Being co-scripted by Joss Whedon, it probably goes without saying that this is very smartly written, possessing plenty of heart and humour, and more than a sliver of satire. And there's a great cast on hand to deliver all the witty banter, such as Thor's Chris Hemsworth and Fran Kranz (from Whedon's excellent series Dollhouse), who plays probably my favourite character in the movie... a pothead whose predilection for Mary Jane seems to give him a more acute perception of the situation he and his friends find themselves in.
I suppose I should briefly mention the basic set up for anyone who is unfamiliar, though I'm sure this is self-evident from the advertising anyway. It's ostensibly a teen slasher movie, but to say any more than that would really spoil it. All I'll say is this... if you have even a remote interest in the horror genre and enjoy seeing it stretched to its sheer conceptual limits, then do yourself a favour and watch this immediately.