I just saw this movie at the theater. Here are my thoughts on it. I went unspoiled (no trailers or anything); if you want to do the same, turn back now.
I was only aware this would be a somewhat metafictional and deconstructed horror movie. I rarely see horror (I had to leave Battle Royale a few minutes in, since then I've seen Cube and the Hole), but was anticipating this because Joss Whedon (he coauthored the script with Mutant Enemy writer Drew Goddard, who directed it).
It was a lot of fun. The intro has some beautiful shots, and immediately gets the metafiction out of the way by intercutting with the facility, which eased my mind; I was unspoiled again. The introduction of Dana, the driving, and the sudden, huge title were cool and pulled me in. The packing scene is fun with a bit of inter-character tension, something that will get rarer as the characters get railroaded into the scenario. The most developed character is also the one I found most sympathetic, Topher Marty whose reefer rants feel way more right than they have any right to be (society not falling apart, but becoming more amalgamated in some sense? there’s an essay there). “I’m going for a walk” was also amusing, because we couldn’t decide if it was reverse-reverse-psychology (not wanting not to be influenced either way by a suggestion) or just suggestion that compelled him to do it. I liked the truth or dare scenes, it’s a nice way to move the plot forward and still keep it character-based. The blonde bleach seemed to be a clever way to lampshade the dumb blonde stereotype and fireproof the script against a reflexive feminist critique; however it may have felt a little too deliberate even for most of the audience. The gore wasn’t too much of a problem, I tend to distance myself with an it’s just a movie mantra, and doing that is especially easy when the movie is so explicitly self-aware. Also the pain-worshipping rednecks didn’t feel too otherwordly, unlike some of the other monsters. I disliked the use of pheromones; having that artifice come from a chemistry department rather than waved away as magic actually makes it harder to swallow. I don’t think we have a pheromone that could urge even a basic “let’s have sex” (as opposed to broadcasting “I’m horny”), and immediately overriding an already decided “let’s stay together” with “let’s split up” is just unrealistic. I’d have preferred a battle of wits and paranoia-inducing mindfucks, perfectly achievable with just audio equipment and a creepy setting (think BtVS’s First, who despite being intangible goes all And Then There Were None on the potentials). On the other hand, the way it’s done keeps the movie fast-paced (it’s just 95 minutes) and fun.
There is constant escalation — from passive set-ups like the halfway mirror, to trapdoors slamming open and doors slammed shut, to the mountain crashing down on the tunnel, to this immense barrier interrupting a clever, daring, and perfectly heroic escape; then the same pattern takes place inside the facility, going all the way to an ending that ends humanity, not even stopping there but showing a bit of what comes next. Some of the mayhem at the facility was like an amped-up version of the fall of the BtVS Initiative. The camera pulling back on all these little boxes of monsters and horrors was reminiscent of the SCP Foundation, an open-ended collaborative fiction project that catalogues this sort of thing.
The harbringer rant on speakerphone shows the staff ignoring a presage that it turns out was directed at them (I mean the general theme of arrogance leading to slaughter; there was also a more specific hint that the fool was not within parameters, also duly ignored). I was rooting for both sides here; there were hints of sabotage at the facility, which dramatically speaking allowed for a conflict between the staff and a shadowy enemy. The One True b!X at Whedonesque has an interesting interpretation, that the facility was condemned not for doing a necessary job (saving humanity is the morally right thing to do by any utilitarian metric), but for being cruel, constructing elaborate scenarios, and generally playing with their food. There were late signs that the staff was reverting to a more earnest stance; the last mid-manager’s last words are “please kill him”, then the director gives an apologetic explanation over the public announcement speakers, and ultimately just makes a request that it turns out was completely honest. It turns out Whedon and Goddard gave away their own interpretation in interviews which I haven’t read yet — according to which they go by a deontic ethic that would condemn the sacrifice either way (strike that, that was second-hand from a comment on the AtWQ review and I didn't find it in actual interviews), and naturally they engage in some navelgazing about the two supervisors being a stand-in for the cineasts — but I prefer this one.
Edit: I've now read a few more reactions to the movie, particularly from that MetaFilter post below, and there's something that helped me make my peace with the ending. The crew are a film crew, the elder gods are the audience demanding certain clichés and some level of exploitation. Both are complicit in a somewhat dire industry (I can't say about the horror genre, though the film does say how it forces what would be intelligent people into stereotypes that don't suit them, and show how its first kill was essentially a sadistic form of slut-shaming); I can say with more confidence, though that's less damning, that the superhero genre is a little too adept at doing movies that will predictably satisfy audience expectations. At that level, it makes sense to say fuck it and let it all burn to the ground; that means the end of the movie world. It also means the undead corpse of the audience rises up from the earth, newly revived, fist raised, left to make its own fun rampaging in a world that won't cater to its expectations.
Recommended: Sure. The crescendo makes this particularly exhilarating. If possible in a form that allows you to pause or rewind; there's a lot of background detail.
Links: TV Tropes — that other wiki — a Tumblr that might be official (a particularly detailed post about that whiteboard) — Whedonesque (latest movie discussion to date) — MetaFilter