This is not exactly the kind of books I usually read, but I was curious - I think it had been recommended at Worldcon last year for one reason or another, and it's been on my list ever since. 
It's not as horror as the blurb makes it sound, but it's still out there. It reminded me of The Cave or The Descent, except not really. It's a gripping read. The main character, Gyre,'s one link to the surface is unreliable. Perhaps Gyre herself is not reliable. Is she really seeing what she's seeing, and what are the implications?  And in some ways you know the characters are emotionally or psychologically compromised, but there's still the possibility that it's all real. And of course, everything is from the point of view of Gyre, who is stuck in that cave in one dire circumstance or another. It felt claustrophobic at times, and hopeful at others, but it was, all along, a gripping and good read. The stakes were kept high throughout as well.
I could empathize with both characters quite a bit, and it was easy to feel for them both, although I think I was supposed to mistrust Em a lot more than I did. The two of them definitely had that "I hate you but you're still hot" vibe from the start, and the dialogue was definitely entertaining!
I did not think, as I was reading, that any ending could really be satisfying, but it's something the author managed to pull off!
The one drawback for me was the map, if I'm being honest. It helped, but it was also not reliable (3D things shown in 2D tend to do that) so it confused me more than it helped, at times. And yet it also did help me get a better idea of the expedition... Anyway, when the map's all you can complain about, it's definitely a good book!