The premise is pretty simple: Your memory has been erased, you are being trained as an assassin, kill or be killed. Along the way there is plenty of action, betrayal, revenge, intrigue, drama, and a small smattering of romance.
The series as a whole is broken up into three arcs with the first two being the most sophisticated, logical, and clever. I'd go as far as to say you even get a bit of the wonderful Death-Note-style intrigue/manipulation in the first arc. Unfortunately all that gold largely disappears in the 3rd arc, where characters often make silly, needlessly complex decisions, arrive at illogical conclusions or undergo unexplained shifts in character. That being said, arc 3 is still entertaining and it doesn't matter because you're already hooked :).
Art: The animation seemed pretty crisp to me. I don't have a background in art or drawing so I can't comment too much on this aspect.
Sound: The music was pretty good as animes go. They play this one song that sounds like "Clubbed to Death". I haven't found it yet but it just has that kind of unrelenting mafia bad ass feel. The other stand out song was Cal's intro song (you'll see what I mean). It's called Battle of Crimsons and I really liked the way it melded piano and kick ass techno beats.
Characters:
The characters were very good. The series kept you fascinated by their decisions and their thought process. It made you ask yourself what would "I" do in this situation? How much would it take to make me kill? Would I behave like them? If you want a black/white, good versus evil kind of struggle this is definitely NOT the anime for you. There is lots of moral grey area, guilt and regret. Character development is strong during the first two arcs but then stalls in the 3rd. Basically it becomes harder to relate to their decisions because many don't really make sense. I'll just stress that overall, the characters are engaging and fun.
Enjoyment:
I definitely enjoyed the series. Even though I felt like they could have done some things better they still delivered. I think it really depends on what type of anime you like.
Overall:
I gave this an 8/10 overall because I think it does have some flaws and unanswered questions. Still worthwhile.
Misc:
Violence/Gore: Lots of gun violence! Definitely not for children or the faint of heart.
Romance: moderate-minimal
Sex/nudity: minimal-none
Consistency: Some inconsistency in character actions/decisions.
Ending: potentially frustrating w/ loose ends
