It's quite interesting. However it's also a bit dry and more interested in societies than in individual characters. I still liked it, though.
The worldbuilding is amazing. It's the whole point of the show. You've got a psychic desert island community, except that their "island" is actually a Mud Whale swimming through a sea of sand. The whale's alive. I'm not sure why it's a Mud Whale and not just a whale, but maybe it's a Gaia-like thing? Even the soil and stones are part of its body? Anyway, the people on its back come in two kinds: (a) Marked, who can use magic called "thymia" and have a thirty-year lifespan, and (b) the Unmarked, who are normal. They're ruled by the elders, who by definition have to be Unmarked, and they've got cool SF culture-specific habits like clasping your hands to suppress your emotions.
It looks like a fairly nice life. Admittedly you'll die young and you're ruled by a bunch of old farts who look scared, wrong and head-in-sandy to me. However the people are nice, the weather's great and you have cute SF animals jumping around and saying "kyuu kyuu".
Unfortunately there are other whales. Some of the people on them are less nice than our heroes.
There's a pretty good story here. Unfortunately it's mostly happening at the level of entire societies, with attempted invasions, genocides, annexations, etc. Individual people just tend to be the victims of this. For example there's some pretty wild stuff going on with the nous, which is something to do with the whales, but no one really seems too fussed about it. They don't ask the obvious questions, e.g. "what the hell?" There's a child soldier thing that just sort of happens.
Significant characters include: (a) Liontari, a camp psycho who LOOOOOVES killing people, (b) a grim rebel called Ouni who just wants to get off the Mud Whale. He's the leader of the "moles" and possibly the island's strongest thymia-user.
...and that's it, really. The ostensible protagonist, Chakuro, is the island's archivist. He keeps a diary. In other words, he follows around the more interesting people who actually do stuff. (He seems like a nice boy, admittedly, but his story role could almost have been fulfilled by a hat.) The cast's big, mind you. It's just that these are mostly people who don't really change the story much. Suou's a well-meaning idealist. Lykos seems like a fascinating character when we first meet her, but she ends up fading into the background. Lots of people die, which obviously limits things. Shuan's memorable in an evil-minded way, though, and Ginshu seems to be having more fun than anyone else (bar Liontari)... so of course she fades into the background too.
It's a good show, I think, but only if you're into worldbuilding-heavy dry SF. There's lots of cool stuff here. I like the ideas. The art is gorgeous. It's an unusual kind of story for anime. It's capable of being shocking. However it's not trying to tell its story through its characters, which I think put off a bunch of people. I'd point out, though, that it's quite good at making its characters likeable when it wants to, even when they're not actually important for the storyline. The Mud Whale people are terribly nice, e.g. not lynching anyone in retaliation for a particularly questionable decision from the elders. (I'll let you find that out for yourself.)
Netflix picked this up, for what it's worth. I wouldn't recommend this to any old random viewer, but SF fans might consider giving it a whirl.