It blew me away. I think it's one of the most remarkable anime of 2017.
It's a tragedy. It's set 500 years after mankind was destroyed, starring the last surviving man (who actively doesn't want to be nice) in a world that doesn't like him very much. The world's full of animal people now. They're furry, or scaly or whatever. You've got talking cats, lizards, armadillos and so on. If they see someone with an ordinary human face, they're capable of turning into a bit of a mob. Our hero's called Willem and for some people he might be a distancing factor in the early episodes. He's capable of being rude, distant and/or condescending. Ask him to be heroic and he'll just say no to your face. In ep.5, he gets his friends saying, "Willem's like a bad guy." This is ironic and painful given his backstory, when you eventually learn it.
By the end of ep.1, the show was looking like the usual light novel formula, but with some darkness in the worldbuilding. Lots of girls! They're cute! Some of them like him romantically! Is this a harem show? Not really, for me, but I can understand how people might call it that. Willem is a withdrawn loner protagonist with no social skills and a dark past who's going to be surrounded by adoring females in a fantasy world! Is he a hidden badass? Do you have to ask?
That said, though, this absolutely isn't the usual light novel power fantasy and I'd disagree with judging it on, say, how well it works as a light romance or on the chemistry of the leads. (Willem doesn't want chemistry!) The world is under attack by "Beasts" and Willem is the caretaker of the only weapons that work against them. These are little girls. You give them swords and send them off to fight to the death. A few of them are fifteen or so, but it's unusual for these girls to live long enough to become a teenager and so most of them are tiny. (I think they're supposed to be ten years old, but they look more like six.) Until Willem showed up, their only caretaker had been a man-eating troll (albeit a beautiful, kind-hearted one who'd never eat you without first getting your permission).
Personally I didn't find the show depressing. Sometimes it's funny, e.g. Tiat's love of lizard movies. "In 'When the Bell Rings Thrice', the heroine cries here, holding her former lover's severed tail!" (That said, though, there's also a darker character point in there with Willem's inability to empathise with lizard romance.) The tiny children are adorable and always fun. I could spend all day watching them rugby-tackle him. There are topless massages (although the camera sees nothing) and a fairly silly bit in ep.5 with the Annihilation Knights.
That said, though, the ending packs a punch. I was a little bit shell-shocked. I wouldn't call it a downer, though, because it's much more complicated than that. There's unexpected triumph and even joy. "I am the happiest girl in the world right now." I'm pretty sure a lot of viewers will have gone away poleaxed, though, and there's no way that's going to be uncontroversial even though personally I thought it had been clear for quite a while which way things were going.
There's a romance. It's unlike most light novel romances, though, because it's going places. It has a clear dramatic shape and real character development. Willem's actively resisting. He's deliberately being hard to like. Two-thirds of the way through, though, we realise that she's teasing him and that she's basically winning. At the same time, though, the tragedy's coming. It got signposted early on, as clearly as anyone could ask for, and from then on you're seeing two layers to all these scenes.
It builds and builds. The more we learn about this world, the worse it gets... but the nastiest revelation isn't presented as a revelation at all. We're trusted to work it out for ourselves. My favourite thing about that twist, incidentally, was how the show's most important use of it isn't for shock value, but for characterisation and counter-intuitive grief in ep.12.
It's complicated emotionally. It makes haunting choices, even including the music. Scarborough Fair, bloody hell. Willem, for me, transcended his theoretically familiar genre role to become a character in his own right, not just a collection of power fantasy tropes. The other characters are... well, I shouldn't say. Spoilers. That was really quite something.
"Long ago, a race called humanity was trying to destroy the world."