It's not great. It's not without interest, but it has periods of drift in the second half. It's about the emotional problems of messed-up teenagers who have to pilot mecha into an environment of flesh-eating death metal infection and save the world from eldritch abominations.
I liked its freaky world. Tokyo is being gobbled up by the Lightless Realm, in which exist things called Corpses and Admonitions. Sometimes they emerge, which is very bad in a manner that I'm still not entirely clear on. Well, they're the size of buildings, they might have been made from the flesh of corpses and they can smash things up. Even if they'd been helping old ladies across the road, I can see how they'd still be unpopular.
The Lightless Realm is definitely bad, though. If you spend too long in there, e.g. thirty minutes, then you're in danger of necrometal infection. There's no cure. Your flesh will turn black and hard. This will eat your body bit by bit until you're just a statue of yourself. Oh, and apparently if you hear the Song of Death, you have nine days until the necrometal consumes you. This appears to be true, at least under some circumstances. There really is such a song. Its most memorable line might be "burn the bones of crying children" (assuming I'm not remembering a different Song of Death) and we hear it several times. It provides spiritual support for one of the main characters.
If you're looking for doom and gloom, this is a good bet. There are also some repellent scientists and politicians, who at first glance just look callous and horrible. They are, but you have no idea. Wait until you hear the stories of Tsugumi and Mimei. The most memorable thing about this show is the horrific, soul-scarring things that were done by powerful men to cause all this. They hadn't anticipated this outcome, but at least one of them is actually happy about it.
Ep.18. Mimei. Poor, poor Mimei.
The show doesn't really have a sense of humour, while its characterisation is quite good, but could hardly be called cheerful. The main character is Akashi, who hates himself for being pleased about his brother's death and has cut himself off from human contact to the point where he'll shove away people trying to get close to him. This can cause his friends to emotionally self-destruct. The most lively, sympathetic character is a massive bitch, although after a while we learn that there's more to her than that. Our team of 'heroes' will include one terror-addicted psychopath whose name is the English word "hate", whom we first see beating a policeman to a bloody pulp before popping the guy's eyeballs with his thumbs. Not everyone's that bad, mind you. Some are kind and innocent. (You're free to speculate about their life expectancy.) There's also a gentle, easy-going chap who... um, spoilers.
I liked the first half of the show. All this is a lot to get used to, both for the audience and the characters. It's disturbing and upsetting, at least when the show gets it right. I liked the self-awareness underneath the bitch's shell, plus the way the characters had to get used to each other (and to losing each other). There are inhuman revelations.
Eventually, though, the show runs out of establishment and the story gets a bit muddy. It doesn't really have villains, as such. It has very, very bad people, but our heroes are working for them to stop the Lightless Realm from eating the world. This would have been more dramatic had the Lightless Realm not drifted into the narrative background. Fortunately we soon learn why all this is happening, which is quite good, but unfortunately the effect is to turn the anime into a giant therapy session for broken people who might destroy the world. This is less dramatic than it sounds. Destroying the world... yeah, well. Fair enough. Go for it. It's the emoting damaged characters who are being foregrounded and they're a bit too dreary and unsympathetic to really get a death lock on our emotions.
They're still fine, though. It's not bad, really. The Minashi-Akashi relationship goes to some dark and unhinged places, saying some very uncomfortable truths along the way. The overall story being told here is quite good, I think. It's just not as involving as it might have been, with key elements of its tragedy being too abstract and SF-handwave for us to be sure that they won't get reversed or retconned at some point. (I'm thinking in particular of the LIM pseudo-death sentence.)
There's a modest amount of what could be called fanservice, but the show's tone makes it as arousing as an autopsy. Tsugumi has big boobs, but she's more broken than anyone. Emiru's bathing scene is when she's being assaulted by the eyeball-squishing terror maniac and EVEN WORSE SPOILER.
The conclusion's a lot less downbeat than it might have been. It wouldn't be ridiculous to call it a happy ending, although don't go expecting singing Disney birdies. In the abstract, I like this show. Much in it is good and even powerful. It's interesting to contemplate. It's just not as compelling as it might have been, I think, with the tone a bit too one-note to really hook the audience. I certainly wouldn't write it off, though.