I love when I skeptically click on these 5.4 rated movies on imdb (what it's at now), but what I get is a 6.3 calibre movie. There wasn't any point in this one where I was like "nah, that wouldn't happen" or "nah, I'm bored." This was just solid all the way through, with no boredom, no cringe moments, no scenes that drag out, just pure claustrophobic terror. I will say most of the jump scares were predictable horror tropes (girl turns around quickly and there's a face behind her, etc.), but they were still high quality for what they are. It's interesting, the fact that something this good only got a 5.4 has me wondering about other horror movies around that score. Maybe there's a lot more good ones then I thought.
Good scares, good atmosphere, good acting, good gore, basically good everything. The only reason I didn't rate it higher is that it's a very simple premise. It accomplishes what it set out to do, but what it set out to do is not very ambitious. This isn't high art, like Event Horizon or Annihilation. It reminded me of The Descent, although not quite as good due to less character development. In The Descent, I really cared for the characters. In this one, not so much.
If you want simple and effective claustrophobic horror, this is it.