If you were to summarize this manga into a few words, it would be "RPG plot". It has all the hallmarks of being an RPG game, from the cruel origins to the breeding ground and even the unrevealed past. The plot is quite standard, the art is simply fantastic, but the characters more or less make or break this series.
[I do not use spoiler tags but I write lines before spoilers indicating it is one]
If you don't like bright I-can-do-it-all MC's to the point of doubting his sanity, you might hate this series immediately from the beginning because a lot of it will focus on him.
I look forward to what the writer does with some of these characters, and I don't write the manga off based of some moronic delusions of grandeur that I am a master of shounen manga and can judge a manga just by throwing out cheap insults. In a nod to some on here, I'm not dumb enough to think it's a good idea to leave a review when I just read 1 chapter and take my review from other people's reviews, and neither am I so self-important that I would spread a layer of mundane sophistry over the sentiment "I don't like it". So with that passive-aggressive remark out of the way, I'd like to address some things more closely.
Character development isn't spread out equally. Some characters will receive dozens of chapters that further the readers' impressions on them, and others will receive four or five and poof we (sometimes both the reader and the writer) don't really care anymore. The dynamic of the MC and his childhood friend on opposite sides brings an actual interest into the antagonistic vampire side, instead of putting all their eggs into the human story.
There is nothing extremely complicated or intriguing about this plot, aside from "what's going to happen next?" When you look at it solely from a story perspective, this story lines up very well to RPG standards, albeit not an exceptionally well done one. One of its more critical problems in this sense is the lack of build-up to certain events. Some events (such as Guren's big reveal recently) has been hinted somewhat sufficiently, but others such as what Owari no Seraph really was, was rather abruptly dumped on the readers. The following is a spoiler for chapter 38: Notably this prompts a jarring response when it's revealed what is happening to Kimizuki's sister.
Some stories are party-bonding time, and most of the time their endeavours lead to battles. This is a rather straightforward storyline when compared to writer's past works. In the interest of avoiding spoilers, this line is a spoiler (Ch14): When MC goes berserk, obviously Shinoa can calm him down by hugging him and appealing to his human side, derp.
World-building though, is pretty great. The art does wonders to this since you can see the detail they put into the design. If you've been to or live in Japan (i.e. Shinjuku, Nagoya), it's actually easier to relate and believe in such a world. The uniforms are sharp and, well, uniform, and the manga puts some effort into showing us what life is like in different factions. EDIT: Actually, it would have been nice to see other human factions and encampments or normal squad missions in play, but considering the pacing of this series, that would probably push the main plot back by 20 chapters. This is why I felt this was more of an RPG story as the settings begs for exploration much like in RPG's.
People say the tone is dark or unstable, but I can't fathom how this is "dark". If this was dark Itou Junji might as well retire. If anything it's extremely optimistic (前向き) how the main party goes about reconciling its issues when confronted with big revelations.
In 40 chapters (each containing 40 pages or more), Owari no Seraph has yet to fall into the realm of ridiculous drawn-out manga. New characters are rarely introduced and prominently featured unless for very specific plot reasons (i.e. one of the Hiiragi, Makoto to create a more realistic and relatable sense of the squads, etc), although it has numerous unimportant supporting cast members. A big problem though is that it suffers from a very abstract sense of powerlevels. For a battle shounen, the readers don't actually have a good idea how strong MC is as compared to this or that vampire.
And to be completely honest, one of the big reasons I've enjoyed this is the action and the 'vanilla' party, which is not to say I find the party bland, but that sometimes I'd like to eat Meiji vanilla ice cream over Movenpick. Some character interactions and relationships are interesting, but the main focus of the series comes back to Mika and Yuu (MC). There is almost no spotlight on the politics that goes into this, as very well it shouldn't, as the story focuses mainly on the perspective of the children. The art helps tonnes and the attention to detail keeps my imagination. The design of the separate factions were well thought-out and actually fleshes the world out in more ways than an infodump ever could. Let me just say that the first thing that prompted to read this manga was Shinoa's smug face so THANK MS. SHINOA.
That's all I can think of really. This is in no way a full review as I'm just shooting out thoughts from the top of my head.
This review was just a gag reflex on the sad and disparaging mightier-than-thou review before mine. I always thought the quality of reviews on this site weren't the best, but that was a new low. Throwing insults and stroking your ego, before finally ejaculating all over light novel authors for no reason. That was a new low, and I hope my review could enlighten you to whether or not this manga is worth your time.
Thanks!
P.S. If you want to make a half-decent review, leaving it short is fine, but you must provide evidence or moments in the manga which will back up your claim. Is the series unimaginative? Bring up other series you think are creative with their interpretation of Shounen cliches. Do you think a blindly-optimistic MC is bad? Tell us the reason why, even if it ends up being "My opinion is that I dislike MC's like this".