Almost there, but not quite.

Rating: 7/10 (19 July 2014)

The first half is actually rather good, playing with claustrophobia and fear of heights until the creatures start attacking, just before the 1 hour mark. Then you realize that the film's antagonists are inconsistent, nonsensical, and incredibly incompetent until the script calls for them to specifically kill someone.
The monsters appear to be some kind of subhuman carnivores (who have been around for a long time, judging from the amount of animal remains and blood we see). They've evolved in the darkness to be blind night hunters - but they suck at using every single sense except hearing. One actually places its hand on a girl's head while looking for her, then starts to move on. Seriously, you can't tell the difference between a human head and ROCK? The monsters also don't seem to have a sense of smell. One of the humans says "they must navigate through sound" but the monsters don't appear to have developed any kind of echolocation whatsoever, which in real life would make a blind predator unable to even navigate the network of caves it calls home. And despite living in darkness, the creatures (1) are shown to hunt outside of their caves (2) don't exhibit the traits of light sensitivity despite their eyes giving the impression that they should. Instead, they seem almost drawn to light, and don't recoil when it's shoved in their faces. So despite being blind, they also rely on sight, but can't seem to see anything properly yes, this is a massive plot hole.
The monsters are literally impossible. There's no rhyme or reason to how they navigate, why they attack the way they do, or pretty much do anything that they do. They're shitty unrealistic villains.
Even The Cave (2005) had better monsters, and those things looked much less realistic. Yet they had echolocation and were fittingly competent within the narrative.
I also disliked the ending of The Descent. I'm not sure if it's a good or bad ending, but my first impression is dislike. There's a theory about the monsters not being real, but I'm pretty iffy about the validity of that considering a sequel was made. At any rate, as some IMDb users have said (see the discussion board for this movie), the monsters sucked and should have been (1) excluded, or (2) completely rewritten. Their appearance was fine - the costume/prop department did a good job - just their behavior was terribly written. Almost every scene with a monster should have been redone.
The Descent could have been a terrific horror film. Instead, it's closer to mediocrity. The film IS still entertaining but the monsters are remarkably disappointing, as are a few other things in last 30 minutes or so.