For some reason, I feel drawn to manga that are very dark and deal with very psychological and philosophical themes. This is a prime example of the how this this genre should be written. All of the characters are very well developed and all the themes are well pronounced.
Although the series definitely moves with its own continuity, it actually plays out a series of psychological analysis all intertwined with philosophical themes. Partially fueled by the very flawed main character, the resulting analysis is always dark and unexpected. And as the main character solves the problems of other people, he then starts to realize more about his own problems.
Which gets to the main point of the series. Hes not really solving other people's problems or learning about other people. Its all done as an act of self realization. And the problems of these other people are really projections of his own inner demons. Which leaves the reader not only guessing what each characterization means about that person, but also what it means about the main character.
My only problem is actually that sometimes I thing the series drags itself on a bit too long and over too dark of subjects without real necessity. Which seems to both shock the reader as well as be thought provoking. But, atleast so far, I don't think it has really detracted from the series.
Overall, its a series I'd definitely recommend to people, over and over.