So you know the story - five young friends go to a cabin in the middle of nowhere for summer fun. Horror, madness and general violence promptly ensues. There's a scene immediately following said cabin arrival that features a mirror. Another scene sees a character drop a weapon in a.....erm....interesting way (really hard to avoid spoilers in with this thing!) Anyway, those scenes directly tell you everything this movie is trying to do. And it succeeds. On many levels.
The writing is classic Whedon. There's not quite so much of his usual wit and funny-talk, but that's probably a good thing since it's an acquired taste. But there are plenty of laugh-out-loud moments, particularly if you like horror movies. On that note, if you're a horror buff (I consider myself to be nearing that title) you will love it! It does for the standard Monster Horror what Scream (1996) did for Slashers.
The characters are deliberately stock. The jock, the slut (not my words!), the smart glasses-wearing guy, the joker/geek, and the innocent virgin heroine. Some time is spent at the beginning of the film to get to know these guys, but it's not really that necessary, since you've seen them all before. Like I said, that's deliberate. And enough twists are given to their personalities to make them fun players (to explain what I mean by that would give too much away).
The aforementioned heroine Dana (Kristen Connolly) would easily fit in any of Whedon's other works - she's one of those Strong Female Characters he loves so much. And I love her too - she's strong, powerful, funny, and capable of stabbing scary monsters repeatedly (a useful trait in any girl!)
Overall, the script is funny and tightly-written, the performances are right as they should be (perfect casting!), the horror is standard but deliberately so, and the twists and turns keep you hooked throughout. I often rate a film based on what time I look at my watch. That doesn't mean I get bored at that point, but I want to know how long there is to go. For example, Captain America had me checking an hour in, whereas Thor kept me hooked until nearly 2 hours. During The Cabin in the Woods, I didn't check the watch once. Yeah ok, I know it's not that long, but it must mean something!
It's a great film, no two ways about it. It's not just a horror/comedy. And it's not just a critique/love letter to the genre either. The film has something to say about the world as a whole, as all good films should. And fans of Whedon's other work (Buffy, Angel, Firefly, Dollhouse etc) will enjoy spotting various actors used before. It's not too spoilerific to say that a certain nerd who features heavily in Buffy season 7 pops up
Joss Whedon and director Drew Goddard worked together for years on Buffy, and it's clear they do so perfectly. So go see this film immediately!