Claustrophobic terror with some heart, and a fair amount of cannibalism.
"Below" should be a fun read for most horror fans. The premise is captivating, the characters are cool and interesting, and the story is well delivered for the most part.
Without delving into spoiler territory, I'll simply say that I particularly enjoyed the treatment of the antagonist, which is fairly unique and nuanced for the sort of tale that's being told.
There were a few small things that didn't completely work for me and a few little details I believe would have benefited from more polish and refinement but I found the overall effort solid and enjoyable.
Bottom line: If you're looking for a quick, propulsive, subterranean creature-feature with some noteworthy elements, I'd definitely recommend taking a trip "Below."