My weird oppinion...
OK. I like this series, mostly for what the other reviewer mentioned (art the feeling of vastness). But here is the thing: This work never felt sci-fi for me. In my eyes it's a Moorcockian Dark Fantasy story covered in a thin layer of cyberpunk. Where the demons of order and chaos wage never ending war while humanity is caught in the middle, because it lost the ability to control either side. And I use humanity in a rather loose sense as there is an "elven" forest here were the Last of Her Kind mad princess rules, endlessly recreating her lost mortal lover (the heroes join this story when the small people *cough*dwarfs*cough* return there). But there is also a scientist *cough*wizard*cough* in his tower whos death wasn't even noticed by his creations, or a Body Snatcher mind saved on a "flashdrive" *cough*ghost*cough*. I think I stop now. So yes. It actually makes more sense to me if I read this series as fantasy, but it might be just me.
First off, the central appeal of this (and the bulk of Nihei's work) is in the art. Scenery gorn and porn intertwine into something darkly beautiful, rife with decay and yet at the same time very much alive. Tunnels, corridors, mind-boggling structures, and the detail will leave you wanting to explore and drink in more and more of Nihei's scenery. The series' atmosphere, while dark, is also sublime, almost. The timescale of the storyline and the sprawling, endless nature of the environment inspires a sense of vastness that stays with you. Nihei's character designs are fairly cookie-cutter in nature for his human characters, but he creates a menagerie of horrific cyborgs and monsters for them to blow up, just as rich and creative as his scenery. Character-wise and story-line wise, we have a bit of a mixed bag. The sense of vastness mentioned above is enforced by the subtle feeling that there is some sort of background story that we only catch glimpses of, which are further elaborated upon in N Oi SE. Whether its vagueness is intentional with sprinkling clues or simply lack of effort on Nihei's part is up for interpretation, but multiple readings of Blame! and N Oi SE allow one to fill in the blanks as they consider the motivations of each organization and the characters. Having said that, the characterization, like everything else, is vague to the point where some may have trouble attaching to Killy, Cibo, and company. While we catch glimpses into their personalities, they are seen more often fighting for their lives and traveling through The City, their dialogue (especially scarce in Killy's case) used for exposition rather than characterization. However, this is not always the case. As said before there are subtle, VERY SUBTLE, hints at their inner workings. Little reactions. Killy tearing into a trader in Volume 2 for instance, or his destruction of a cloning machine. The plot is, in a word, simplistic. Point A to Point B. The Quest. Overall, Blame! is but a taste, a brief glimpse, into a vast world, and definitely worth visiting.