Ahhh i don't want to be one of the few who were slightly disappointed with the show overall, because i can see so many extremely enthusiastic comments.
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For those worried about the violence, here's all you need to be wary of(spoiler free, without naming anyone):
VIOLENT CONTENT:
A character's head is cut off, and the camera stays on the decapitated corpse. The camera lingers on all the mutilated bodies. Blood pours generously over everyone and everything. Characters are repeatedly run over, stabbed with knives/needles, hit with clubs, mercilessly, recklessly shot(all kinds of guns, from pistols to sub-machine guns), electrocuted, drowned, rammed into walls, bones broken/dislocated. Many of the murders are done callously, with no real time to register shock or emotion. A character who can heal himself is subject to every kind of graphic violence imaginable, and cuts open his abdomen to make a point. An eye is squelched out. Limbs are blown off. Hands dipped in boiling oil. A skeletal corpse, people imprisoned in dark dungeons.
Multiple suicides by jumping/falling off high places.

AGAINST MINORS:
Scene where children are being abused(Violent, not sexual) by men in military uniforms/skin branded/shot at/made to kill each other(not shown in detail, but you can gather the idea).
Students are subjected to bullying. Extreme 17-to-1 catfight near school premises among female high-schoolers where the intention is clearly to kill(Weapons include a club and a retractable blade).
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Just finished watching, I'm unsure of how exactly I feel... the production was obviously freaking awesome, the cast did an amazing job with their characters, the plot was intriguing and full of action. Sometimes they dragged out scenes to heighten tension, but there's a line between tension and plain boring, and they crossed it a lot. The VFX and screenplay was absolutely stunning, but all the death, gore, and gratituous, callous violence got to me after a while. I thought I'd be scared or traumatized, but I got sick of it after a while. They use a main character with regenerative powers(Jang Juwon, played by Ryu Seung Ryong) to inflict and sustain every type of horrible violence imaginable. The characters I actually liked didn't get enough screen time(except Han Hyo Joo, who was amazing, as always). Zo In Sung was my favorite, playing the cool, badass, flying superhero so so well(he was also the least violent but smartest good guy, so go figure). Everyone's back-stories are beautifully written.

The kids storylines were initially a big snoozefest, but got better later, around Ep 7. Again, my favorite character, Lee Ganghoon, played by the cutie rookie from The Law Cafe, didn't get as much screentime as I would have liked, and he was the one with the most layered, intriguing personality. His story with his super-strong, mentally impaired dad jut kept bringing tears to my eyes. I would have had Second Lead Syndrome if this was a classic Kdrama, but the romance was the least interesting thing about the kids anyway. I didn't have the time or patience to watch them fall in love, and every moment was excruciatingly slow.

I'd recommend watching it for the plot and production, as long as you're okay with constant, extremely graphic violence. Many, many characters die gruesome deaths, with the camera lingering on their grossly mutilated bodies, but we don't really get to know them well enough to care. It just highlights the absolutely tragic pointlessness of war, and how it's the common people who always suffer most.

They tied up the ending for the main characters pretty fine, but hint heavily at a season 2. Lots of open endings with the overarching crime plot/villain-scheming...
I feel a sense of fatigue with all the repeated violence, and the attempts at bringing heart-rending scenes kinda fell flat most of the time, imo. I only really cried once, and that was when a bunch of young ordinary soldiers(none of whom we know and who we never see again) die in an NK-SK superhuman scuffle, and that's because I cannot stand such scenes no matter what show. Watching high-seated officials send young boys into meaningless death and destruction makes me see red and bawl my eyes out.