This show has an admittedly cool and unique plot. However, the whole series focuses on a kid who is basically a pervert, a loser, and a coward. (Much more so even than Shinji Ikari from Evangelion.) I am not usually made uncomfortable by graphic sexual content, but even I feel that this show went way too far at times. The main character is essentially a deviant, and he seems to be continually rewarded for being a weird little P.O.S. He shows a little bit of character development towards the end, but overall he is still kind of a sick weirdo. There are just some things people don't need to see.
In spite of my opinion on the graphic material, that is not the reason I took a couple points away from this show. I think it should have been about half as long as it was. There were a lot of episodes that were filler material, or just people talking about their feelings. As a personal preference, I hate when long periods of time in anime are spent with people expressing their opinions on things or thinking about their feelings. Meanwhile, there is action that is supposed to be happening in a matter of seconds - but hours are spent focusing on what everyone is thinking or feeling about the situation.
I suppose part of this show's purpose was to explore the darker side of humanity. You end up feeling sorry for quite a few of the characters, and some of the things that happen to people are just downright horrible. This isn't necessarily a bad thing if you don't mind depressing things. Again, as a personal preference, I would rather see something less depressing, but that's just me. Be prepared for gratuitous violence and sex, and seeing how selfish some people really are.
Lastly, I hate when Japanese shows take completely unrelated plot lines and turn them into an "America is Evil" speech or something. The little white haired kid that is a complete psycho basically states that his behavior is acceptable because America does whatever we want. Umm...huh? Think what you want, but don't blatantly roll your political agenda into something that is supposed to be entertainment.
I will say that the plot was a unique concept and it was cool how they tied all the characters to each other. Also, the show was not afraid to kill anyone off no matter who they were. Usually there are characters who seem to be impervious to death by virtue of their importance to the plot. The action scenes were very cool and bloody, but way too drawn out because of the constant "let's talk about our feelings and opinions" breaks. This series is worth seeing, but just be prepared for the content, and to be alternately bored and entertained.