Story: It starts off with six teenagers being inducted into a boy's juvenile prison around ten years after World War II in Japan. They join a guy who has already been there for two years and share his cell. After some initial hostility, they all become friends and together survive the extreme harshness of the juvenile school.
And when I say 'extreme harshness,' I mean crippling beatings, forced starvation, and sexual advances by an old creepy guy. So without giving anything away, the first half is how they get through all of that in the juvenile prison and enter the real world, with strong aspirations to do whatever it is they have their mind set on doing.
The second half of the show is all about them coming of age and starting up in real careers, whether that's what they had planned on or not. The second half starts off with them planning revenge on the people that had tormented them in the first half, but after that, it's mostly a bunch of little mini-arcs, each centering on different members of the group. Time and time again their friendships and wills are tested, and they always pull through for each other.
Animation: I love the animation style of the show. It's gritty and dark in places, especially in the first half of the show, but the scenery and the character designs are amazing. As weird as this is to point out, I love the detail that scars and old... ness... are drawn. And the scenery in the 'touching' scenes is just absolutely beautiful.
The bits where the narrator pauses it on a still get annoying after a while, but the filter effect that's put on top of whatever still always looks pretty cool and makes whatever the still is, no matter how dark in nature, look pretty.
Sound: I'm not much of a 'sound' girl, but the voice acting was great (... except for the Americans, who all sounded vaguely Australian). I can instantly tell which of the group was speaking just from hearing the voices, and the creepy people have suitably creepy voices.
The OP was fine, nothing extremely mind-blowing or anything, but they did make good use of the instrumental in the last episode. The EP was pretty, but once again, nothing mind-blowing.
Characters: Everyone in the main group has a deep and thought out backstory, filled with reasons why they were there and in some cases, why they're distrustful initially. Well, mostly... Cabbage and Solidier always felt a little less thought-out to me.
Mario and Turtle get the most focus in the second half, with one of the two always in someone's else's arc as the support. Everyone else gets a wrap-up arc and a nice epilogue, except for Solider, who they basically go, "He became a solidier in the SDF like he wanted." At the very least for everyone it says what they did to get in the prison.
Overall: To compare it to the manga, the manga goes a little more in detail with the squicky parts (... like the anal examination, you're welcome for that visual), but it also gives a little more character development. Whether you read the manga or watch the anime, though, it's an awesome anime with a lot of substance.