I agree with the below reviewer that there is no reason why any normal person would enjoy this. Normal people tend to prefer safe and predictable things that make them feel safe and predictable emotions. But for those of you who like to have the boundaries of your emotions stretched into new and often uncomfortable shapes then this is a manga worth investing in.
At it's heart it is a tale of finding humanity and while that is not alone worthy of note one of the strongest points is that the villains progress from emotionless psychopath to feeling psychopath to finally an incredibly evil but recognizable human being is almost as, if not more interesting than the protagonists journey from scornful and aloof to desperate and vulnerable.
At times it skips over the line from borderline pornographic to pornographic, but the context is always disturbing enough that it leaves a distateful sheen on your tongue. I suspect that this is part of the intent but either way, expect that you are going to see alot of shit that is going to make you deeply uncomfortable and not only in the category of sex but in terms of violence as well. Bad bad things happen to people and not all of them really deserve it.
Also, despite the often trasgressive content the moral questions and philosphical debates posed by the series are relatively complex. For more adult than many works of ostentibly greater maturity. Especially interesting are the questions that it frames about non violent resistance and the insidious nature of pride.
A final time I would just advise people that if you are the sort of person who has ever put down a book, comic or movie in disgust, not because you thought it was bad, or that it had some message that you found unpleasent, but because it was simply unbearable to continue, this is most certainly not the manga for you. However if you are the sort of person who read and enjoyed works like Berserk or the First Law Trilogy then you might find that there is something worthwhile.