I don't understand why the overall score for this manga is so high.
Artwork
The artwork is below average, nothing special going on there... true, it get's better over time, yet it still remains subpar. Especially in the first volume facial expressions were a luxury.
Fanservice
Since I have a general dislike of fanservice of any kind I won't bore you to much with this. Simply said, there is too much senseless nudity completely unrelated to the story and it is adding nothing. Is it too much trouble to simply slap some rags on those nude 'slaves'. Clothes do serve a function you know... And not just for modesty.
Characters
I'm assuming the 'protagonists' of the story are Kouta and Nyuu since those two are the most recurring and appear to be the 'good guys'. Sadly, because they don't get enough 'screentime' and give any sense that they are 'telling the story' or the 'focus of the story', I'm unsure about that.
There are lots of references to Nyuu/Lucy and most flasbacks involve her, yet no explanation to who she is and more important why is fore-coming. Knowing more about her past does not change my impression or understanding of her.
Also, as also mentioned by others, there is pretty much no character development and any changes in the characters are brought through a convenient excuse called 'split-personality'.
The antagonist is nothing special: cruel, twisted, makes no sense, has a superiority complex and primarily uses minions to do the work (which all pretty much fail horribly and get killed).
Depth
There has been some attempt to add depth to characters (mostly through flashbacks)... alas, this changes nothing and does not help to 'understand' the characters better.
Story
The story itself is pretty generic and offers no originality
Yet another 'Kill all humans, create a new race and rule the world' story
Also, no explanation about what these so called 'diclonius' actually are, where they came from and how the world is reacting to them is given... Which would seem pretty crucial to me in understanding the story, some of the plot twists and ultimately the characters.
I understand this manga focuses mostly on gore and violence... Yet no reason why is offered. (and please stop with the sappy/crappy 'cruel youth' flashbacks). If the whole 'kill all humans' part is justified by a few flashbacks that indicate bullying in the past I'm even more disappointed, this could have been done much better.
All in all, waste of time that could have been much better spent... like by banging my head against the wall, same degree of entertainment (none) and annoyance (much) but at least I'd have gotten some exercise.
If you like Naruto / Bleach / One Piece or any other of those senseless shounen manga's this is probably pretty much up your ally... at least whilst waiting for the next chapter of any other manga.
If, like me, you don't. Just skip it... there are many other manga out there that at least offer some sense of fulfilment.
You still want to know what happens and what not... Watch the anime, or skip random chapters (seriously you won't miss anything).