You deserve a review that doesn't ruin any of what's coming in this incredibly fun, surprises-around-every-corner movie.
So I'm going to assume that you haven't heard or read a thing about it yet. I'm also going to assume that you haven't seen the ambiguous trailer, the one that still kind of gives away too much stuff, and that your jerk friends haven't gone to some brand-awareness sneak preview and destroyed it all with enthusiastic blabbermouthing. And if you already went to IMDb to look up the cast? Well, then the joke's on you. You've spoiled a bit of it for yourself, all by yourself. Sorry.
I'm going to assume that all you know about it is the most basic premise, that five college students drive to a remote location to party all weekend long at a cabin in the woods. And then bad things happen to them. And it's great.
That's because it honors its responsibility to horror films even if it doesn't take itself seriously at all. After disgruntled Star Wars people (whose hatred of the thing they love has no rival), horror fans like me are always complaining about horror films. We wait around impatiently for filmmakers to do the genre justice. We feel cheated when they don't. But what we're especially waiting around impatiently for is a film that twists the genre into a new shape or at least turns horror conventions on their head for a bit. New shapes are rare, like finding needles in haystacks covering up trap doors that lead to blackened pits housing redneck zombie torture families, so don't get too excited. But this movie has the poking convention with a stick part down cold.
It's a horror comedy that's funny but isn't always going for the joke like, say, Tucker and Dale vs. Evil (another satisfyingly unusual entry). And it's suspenseful but not exceptionally scary, in the manner of a very bloody episode of The Twilight Zone. In fact, unless you've never seen a horror film before you probably won't be scared at all. But then again if you've never seen a horror film then this isn't where you should start anyway. It relies on you pre-knowing a lot, starting with the Scream-exploded "rules" about the priority and order of life and death in a scary movie. It assumes you're already devoted to the idea of a fresh take on disregarding those priorities. It demands a viewer with a compatible level of self-awareness. Think Scott Pilgrim vs. the World, set in a slaughterhouse.
Which means it's show-offy, too, altering traditional scenarios and smashing them together to create brilliant moments of confusion before resolving... well, I've actually said too much now. But this is part of the deal with Joss Whedon -- working here with Cloverfield-creating/Lost-producing Drew Goddard, thereby infinitely multiplying the film's "look how clever we're being" quality -- a man who's skilled at taking an old story you think you know, then elbowing you in the side a bit, delighted with himself, as he post-post-whatevers it into his own cover version. Then when he's done he tells you, "You're welcome."
A little annoying, maybe. But the guy earns it. Go.