This is the kind of manga that you definitely shouldn't read if you're feeling depressed. It's a cold, hard, unsympathetic and merciless look into the cruellest, most selfish, most twisted parts of society and human nature.
Iida is the kind of personality you rarely see in fiction, especially as the protagonist. He's icy-cold, strong-willed, calm, pragmatic in the extreme, and 100% sure of who he is and what he wants. He's not a likeable man, but imo he isn't meant to be, nor should he be considering the sombre tone of the plot. It may just be my fancy, but I believe the mangaka intended for him to be the embodiment of justice (or the Japanese ideal of it), and his character truly reflects that.
I thoroughly enjoyed (though 'enjoyed' may not be quite the right word) his relationships with his co-workers and acquaintances. Of particular note are his interactions with his second partner, Eriko: it was completely devoid of romantic tension despite their obvious mutual fondness; because workplace romance is completely out-of-character for both of them, the author had the backbone to keep it out of the story, though it undoubtedly lost him quite a few readers in the process. So if you're looking for a love story for either of the MC's, it's probably best to stay away. (Eriko has a love interest, but it is not the focus at any point)
The episodes are often very extreme and not particularly feasible from a real-life standpoint, since most of the cases (not to mention Iida's methods of dealing with them) are pretty fantastical; but that's actually a good thing in my eyes, as real-life crimes tend to be dull, uninspired, and lacking in suspense. But the actual emotions (lust, greed, hatred, self-preservation, religious fanaticism, mental instability) behind the characters' actions are very real indeed, and that's what matters to me personally when it comes to crime fiction.
The art style reflects the mood of the story perfectly. It's neither aesthetically pleasing nor 'sexy,' but I personally found it appealing in its own odd way.
Recommended for the jaded reader who's ready to handle a less "flowers-and-hearts" outlook towards life.