I don't know if Cabin in the woods should be considered a spoof or not. Typically a spoof is like a funhouse mirror; it shows a distorted reflection of the subject being spoofed. Ideally this is a commentary or analysis; rather than an essay that dissects the shortcomings of the subject, the plot flaws and tired tropes, it simply makes them more obvious so we can identify and recognise them ourselves. Of course, sometimes it's nothing more meaningful than a funny reflection.
Cabin in the Woods doesn't do that. Rather than just parrot back a generic backwoods horror story in a funny voice, it writes a new story. A typical spoof would tell us how overused these horror story elements are, Cabin does so while actually still using them - and using them effectively too. This arguably makes it a homage rather than a spoof.
If you consider Cabin in the Woods a spoof, then it's an intelligent one. If you consider it a homage, then it's a very original one. Regardless, it's a fun movie with a clever plot. It kept me guessing right up to the very end, which not many movies do these days.
I give it a 9/10; fun, clever and original. Note that there is a fair amount of blood and gore and a little nudity. You have been warned.
#####SPOILER WARNING - SERIOUSLY, I MEAN IT!#####
I love how drugs cause the end of the world.