When I saw this movie, I was the only one in the theater except for some people in the back. I was in the front row. During the movie, the entire time they were in the shimmer, I was captive.
[SPOILERS]
First off, the plot. Slightly loose, but the visual and hidden details pulled it together very cleanly. The calm, yet tense moments in the shimmer were the best of the movie. The military base scene is a prime example. They walk around in what used to be a base for others who planned to go in the shimmer. They don't find much, but what they do is frightening as all hell. One of the best images in my mind is of the skeleton that was merged with the colorful plant on the wall. The upper body was ripped from the lower body, and the skull was ripped off and facing outwards just above the upper body. But still, it was calm somehow.
Second, the scarier parts. The bear scene specifically. A mutated bear kills one of their crew. It returns mocking the noises she made as she died to the bear's hands. Absolutely terrifying scene.
Finally, the soundtrack. This wrapped it all together. The songs were simply played on an acoustic guitar. But they were the things that made the movie so well-made. It makes you feel calm while you see something that is straight out of your nightmares.
10/10. See this movie if you have not.