Short Version: FANTASTIC!
Medium Version: Five friends go into what is supposed to be their certain predestined doom, only to find out that reality is often messier than the fictional fiction they are supposed to emulate. Which might sound crazy but so is the movie.
Long Version: Spoilers below.
So this is basically a meta movie. I've read some who call this an analysis of the relationship between the audience and Hollywood. There is some of that. After all, horror fans demand these conventions and are squeezed into amalgamated cans of preponderated crap that are then packaged and sold to the Sline populace for as much bang for their buck as possible on Opening Weeekend.
But I think it is more than just a meta movie. It is also a movie that tries to apply reason where reason normally has no business existing.
Why do ordinarily smart people split up or have sex or mess around with the creepy artifact? Are they really guilty of something when the odds are tilted in the favor against them so much that they are doomed from the start?
This movie is on the surface about five characters in search of a bloody death, but becomes much more once they get out of the cliche and into the support structure of the infrastructure. Joss Whedon both obeys and destroys regular convention but does so in a way that still keeps it grounded in some kind of reality without going full out Monty Python's Quest for the Holy Grail.
I've heard some people hate this movie but many love it. If you like Whedon, odds are you'll like this. If you hate movies that make you think, even if you normally like 'all kinds of cinema' then this probably isn't your thing.
Me, I probably will own this movie even if I don't normally like Horror per se (though I am starting to like it.) I like things that break the norm and break the rules, which this definitely does without getting to wrapped up in its own cutesyness.