I rate this a lot more than most people do, because I like the ending. I actually had to go and do some googling to work out why everyone else hated it. I think it's interesting, although in fairness I had the advantage with ep.13 of knowing that I was watching something that was generally reviled and so was able to set my expectations accordingly.
It's a surreal, delicate, somewhat shocking series. It's about girls who commit suicide. Every week, our heroine Ai fights impossible monsters in a dream world to save another girl who killed herself. It's unclear if this achieves anything, since the girl will inevitably disappear in a puff of smoke afterwards. She seems to be still dead. If something magical happened there, we never hear anything about it. Ai's doing this in order to resurrect her best (and only) friend Koito, who killed herself before the series began and Ai wants to ask her why. She'd also been in a bad psychological place before Koito showed up. If Koito had asked Ai to die with her, what would she have done?
Just focusing on outcomes, though, is a mistake. The journey itself is healing Ai. She fights to protect people. She gets stronger. She meets other girls who are doing the same thing and makes new friends. They're a bit of a problem bunch too, mind you, with a robot-like genius, a handsome girl who's only ever seen as a boy and an ex-idol who treats other people as walking wallets and has repeatedly attempted suicide herself.
They're doing this at the behest of two tailor's dummies who sit in a dream garden and have a sexist and over-generalised conversation about suicide in ep.4. Nonetheless, they're the ones who are mobilising Ai and the others to save all these girls who might possibly have or will have committed suicide. Unless it's a parallel world. And their backstory in ep.11 will turn your spine to ice.
The show's exploring different motivations for suicide. Each episode will give us a different girl who killed herself, with her own unique reasons. The jolly fangirls in ep.3 are incredible.
This show is a fascinating work of art, with extraordinary visuals and high quality animation. (Unfortunately, the small and relatively inexperienced production team couldn't do this while hitting their weekly deadlines, so ep.8 is a recap episode and the show's finale was delayed for three months and broadcast as a special.) It does, though, make some decisions at the end went down badly with the audience. There's a baddie who's been doing impossible but really bad things... probably. In some completely unexplained fashion. It's not clear. She has sidekicks. No idea who they are either. Ep.12 ends with the story clearly unfinished and very little conveyed to the audience on how, say, that baddie fits into the regular dream world fights we've been watching. Our heroines certainly haven't defeated anyone or stopped the baddie from continuing with that Temptation of Suicide, however that works.
...and then ep.13 sorts out none of that. Again, it's about the journey, not the arrival. It's about what the girls did next. (Well, except for one. Spoilers. I'd be keen to see a Season 2 that followed up on that.) They've suffered trauma and they've been forgotten. They go on with their lives, get older and drift apart, because that's the kind of thing that happens. I wouldn't call it a downer ending, because she'll end the series with a new goal and a new mission, but it's definitely not what people had been expecting. Personally, I'd been expecting a Bad Suicide Ending, given everything the show had been talking about, but no. It's about living, not dying. Staying alive means growing up. Ai will become the Warrior of Eros she'd been talking about. That's part of growing up too.
It's deep and dark, but also bright and entertaining. It's tackling horrible material under a veil of fantasy and being extremely weird. I like the characterisation. I think the writing's fascinating. I think I've just talked myself into giving this a rewatch one day to see what I think second time around.