I was surprisingly bored.
As was I. I also found the so-called twist (is it really a twist when it's obvious from the beginning?) to be nonsensical, pointless, and poorly conceived. It's amazing how before we had all of this ultra super technology, ritual sacrifices were pretty simple and straight forward. But now we need force fields, underground lairs, and ridiculous ''rules'' (that seem utterly pointless when you're forcing the outcome anyway) in order to get the job done. Nevermind in the end they were just trying to shoot the stoner schlub straight up.
Regardless, I'm not sure how such a "brilliant" twist is supposed to make this movie any different from any other boring horror flick that changes gears in the final act.
Worse, it ends with two idiot teens deciding that the actions of a tiny handful of people mean the entire human species should be wiped out. Who cares that their actions really did
have a purpose regardless of how poorly conceived that purpose was on Whedon's part? The world only existed because a few people had
to die every so often to appease these all-powerful uber gods. That's a simple fact of this setting. But because these teens don't like that they're the sacrificial lambs, they're not only going to commit suicide (???), but they're going to take the entire planet with them.