Amazing!
The only thing I would have changed is, not showing the creatures, by the way, they didn't look scary at all, I was kind of disappointed on that, just the mistery of not knowing what the guy is seeing is freightening alone, like Signs of M. Night Shyamalan and Mel Gibson, It was scary to hell because you can't see what the characters see, just shadows or blurred images running away.
That's the only thing I didn't like, but the rest was pretty intense.