Links: TVTropes, Wikipedia, IMDB (7.3).
Well, that was an interesting take on a horror movie. Unfortunately, I can't really say much without spoiling everything. It's basically a deconstruction of slasher horror films, but not in the way you might expect: Scream, for instance, simply parodied the slasher genre conventions for laughs; this film instead asks why the genre conventions exist in the first place. And while this film has its morbidly funny moments (two words: happy frog), it also has entirely too much existential dread to qualify as a "horror comedy". (If you don't mind partial spoilers, you can watch the trailer. I recommend stopping at 1:38, when it says "THIS APRIL".) The film is co-written by Joss Whedon (who's having a good plot day in addition to the snappy dialogue one expects) and Drew Goddard (most notable in this context as a writer on late-season Buffy and Angel; faint praise, I know, but he did co-write Conversations with Dead People, the best ep of season 7). If you like Whedon and you like horror, this is a must-see. Score: 4 stars out of 4.
P.S.: If you don't mind moderate spoilers, here's a pithy description in ROT13: vg fgnegf nf Sevqnl gur 13gu zrrgf Phor, gura gheaf vagb Erfvqrag Rivy zrrgf Gur Arireraqvat Fgbel.
P.P.S.: If you have seen the movie or don't mind major spoilers: gur napvragf ner hf (bhe pbyyrpgvir vq); bhe vzntvangvba pnhfrf gurve svpgvbany havirefr gb rkvfg; gur evghny pbzrf sebz bhe guvefg sbe fcrpvsvp ubeebe zbivr sbezhynf; gur snpryrff betnavmngvba pbaqhpgvat gur evghny ercerfragf gur zbivr fghqvb flfgrz; gur napvragf evfvat ercerfragf gur natel zbivrtbvat choyvp erwrpgvat nal zbivr gung qbrfa'g fngvfsl gur penira oybbqyhfg va gurve vq.