Since this is the manga, it's still ongoing. The 2014 anime covered the first 100 chapters or so. One-line summary: at it's best, it's horrifying, but over time that's worn away until by now it's practically cosy.
The premise of this manga is that aliens are real and that sanity-shatteringly bad people have been kidnapping children, performing experiments on them and killing them. It's impossible to overstate the horror of this. They see child murder as standard operating procedure. If one of their girls escaped, they'd kill anyone she'd happened to meet in the street. The girls have superpowers, but:
(a) They have a death switch on the back of their neck, which can either be triggered remotely or by anyone standing behind them. It's called "ejection". If this switch gets pressed, then the girl will melt into a steaming puddle.
(b) The powers often have a price. Every time Neko uses her powers, she permanently loses some memories and can no longer remember her parents or anything about her childhood. The gift of precognition appears to cause nervous system failure, full body paralysis and/or death.
(c) The girls can hardly remember a time when they weren't being used as disposable raw material for torture or experiments.
(d) The girls have to take a pill every day, or else they melt. It's like ejection, except that it'll take several hours for your body to dissolve rather than a few seconds. Unfortunately the girls only have enough pills for a few days, which can put extreme strain on friendships. If one of you died, the other could live a few days longer.
(e) Something worse. Spoilers.
In the manga's early stages, the sense of threat is almost overwhelming. I don't know if I've read anything else like it. Anyone can die at any time, at the drop of a hat... but more shocking than that is the girls' acceptance that their future can be measured in a handful of days. Don't make arrangements for Tuesday. You'll have melted into bloody gunk by the weekend. Ah, good point. How about Friday morning, then? If the weather forecast's bad for the rest of the week, then that's the rest of your life.
The manga's male protagonist, Murakami, tries to help, but for the girls everything's always one day at a time. You're always aware that nothing is out of bounds. If soldiers burst in and machine-gunned the entire cast with no warning, ending the story in a couple of pages, it wouldn't even be surprising.
However our heroes don't die. (Not permanently, anyway. Superpowers.) Then they keep not dying. We start getting a little more comfortable. The danger level drops still further when Hatsuna shows up with her regenerative abilities, although that's not an all-conquering get-out since only one person can be brought back in any 24-hour period. We start to assume that the regular characters will stay alive, whereas earlier we'd been assuming the opposite. The manga remains brutal and horrific, but it's no longer scary. The earlier chapters, in contrast, had been terrifying.
There are also a couple of problems. One is fanservice-based harem nonsense, while the other is Neko.
The harem stuff is annoying. Admittedly it's logical and even emotionally traumatic for these girls to be so needy and unbalanced, but it's still annoying for how blatantly the author's playing on tropes. It's harem bollocks. It has lots of scenes of girls in the public bath. It's a group of girls all in love with the same teenage boy, some of whom keep throwing themselves at him sexually and yet he never takes advantage. (Okamoto doesn't even really try to justify this. We could perhaps assume that Murakami's still obsessing and/or guilt-tripping over Neko, or else perhaps that he doesn't want to tear apart the dynamic of this 'family' by thinking with his dick. However all of these would be fan rationalisations for something that's not in the text.) Sometimes it manages to be heartrending, when we get inside a particular girl's point of view, but often it's just yet another plod-through of manga/anime cliches.
The other problem, Neko, is the main girl and possibly also Murakami's childhood friend. Unfortunately her powers give her amnesia and, in sufficiently extreme situations, complete brain-wipes. Neko Mark 1 is boring, but sympathetic. Neko Mark 2 is a bitch and it's unpleasant to be with her.
The other girls I liked, though. Kana at times comes across as the most important character. It helps that she's too young to be a romantic interest, which cuts her out of the harem nonsense. Skadi's sweet and her arc was skipped in the anime, although don't expect it to end well. Kotori's lovely too. Nanami blew me out of the water. Kazumi and Hatsuna are both sexually aggressive, sometimes to unpleasant degrees (molestation of a paralysed girl, anyone?), but I think the idea with both is to create unsympathetic characters and then find the humanity in them. This works, actually, although they can both be pretty hard to read about at times. Okamoto has a knack for creating characters with the ability to be by turns either sympathetic and moving, or else alienating and creepy. See Takaya, for instance. He's funny, but eyaaaah.
At its best, this manga is shattering. As an exercise in all kinds of horror, it's hard to beat. At what it does, it's brilliant. I also like the way Murakami's cleverness actually is clever, with some plans and logical inferences that are genuinely cool. However the manga also has ugly lumpen clay feet in the harem stuff and Neko. I presume Okamoto's got plans for the latter, though, so he might yet manage to redeem her in later chapters. I hope he does. There's too much power and terror in this manga for anyone to want to see it go to waste.
"Sorry about going after you like that. It must have been pretty annoying to be hounded by a girl you have no feelings for. I'm done now. Don't let it get to you."