We're just back from it, and I spent the entire thing just about not hyperventilating, because I am _rubbish_ at horror movies. But I still enjoyed it massively, loved the way it was put together, and think it's well worth seeing.
I shall say nothing at all about it, because frankly discovering what's going on over the course of the movie is where most of the fun is (along with spotting references to movies you have previously seen). The first two thirds are merely good, meta, scary, fun. The final third is just epic. If you have a fondness for horror movies then go see.
(Feel free to have spoileriffic conversations in the comments.)
Original post on Dreamwidth - there are comments there.