This was an interesting watch. It's not a masterpiece per se, but it's polished. I was intrigued throughout the entire season. Production I.G. is such an amazing studio that I doubt that this anime would have this level of critical acclaim if they weren't involved with the project.
What the show does well is it doesn't really tell you what the hell is going on. It's set in this post-apocalyptic world where a vast majority of people in Japan have been seemingly wiped out. Monsters or Man-Eaters roam the surface and love killing guys. There are remnants of the previous world, but most of it has been either destroyed or repurposed to serve a different function in this new wasteland. I really appreciate when a piece of media doesn't have characters that spew long monologues and drivel about exposition or world building. You sort of just get the gist based off of the actions of the characters and their environments. There are some instances where characters explain bits and pieces, but the main puzzle of this New World remains largely unsolved; sometimes when a question gets answered it only leads to more questions.
The main characters are fine. Kiruko is my favorite of the duo. Their history in the show, without any spoilers, is the most captivating aspect of their character. Maru is kind of whatever. I find him to be a little too shonen-y. He's got this unexplained physical prowess as well as a superpower that allows him to kill these so called Man-Eaters. I think my main gripe with him is that he's just a little bland. He's just a kind of horny teenager who's in love with his senior, and that's pretty much it. Maybe when his history is explained in future episodes I'll come to like him a bit more, but for now he just exists as a guy that I would've loved when I was 12 years old.
I think the strongest aspect of the show in terms of its narrative was the second plot running in conjunction with our duo's story. I found myself much more interested in the relationships between the characters in the compound more than I did on the 'outside.' It reminded me of Promised Neverland so that's probably why I was a lot more engaged. The 'kids discovering the truth about the outside' trope scratches a certain itch in my brain that dilates my eyes and focuses all of my attention on what I'm watching.
All in all, it's a pretty good show. Some episodes stand out more than others. One of them is particularly well directed and animated, with a killer side story and characters. If you love a good mystery with sci-fi elements, give this one a shot. Just know that the questions you have while watching the show will keep piling up and a large portion of them will go unanswered. I liked it, I'm definitely going to watch Season 2 if it ever comes out.