Plot/Story
The story centers around two newbies at the Narcotic Control Department in Japan, Hal and Kai. While struggling agains drug dealers, gangs, murderers, psychopats, misteirous organizations and mad people in general, they have also to deal with their own misteries, specially Kai. The gentle and caring boy, who suffers thogether with the drug users and tries his best to stop people from suffering, just turns into a killing machine suddenly in some cases, like the occourance of a certain event functioned like a switch. What's behind this? Who is he, in fact?

Characters
The characters are DEEP. From the main characters to the random extra n#1 that appeared in chapter 1, not a single character is simple. Everyone has its own reasons, and even the 'bad guys' aren't evil because 'they're'. The authors do an excellent job showing what's wrong and what the 'bad ones' did wrong, but also showing that they were wronged once, or that they have also their own problems to deal with - they just chose the wrong path, the drugs.
The 'good' characters also have their flaws, and it's very easy to relate to them (or even with the antagonists).

Drawing Style
The art is absolutely gorgeous. Something I find very unique is that the authors barely use screen tones, showing their world only in black and white. There isn't also hatchcrossing. Toning is added only, for exemple, when they're in a dark place, or when they need it to show some kind of emotion.
I've got only one problem with the art: I find very difficult to tell some characters apart, since some of the faces look the same.

Enjoyment
I have the hots for this manga. It has everything I like. Good plot, deep characters, good art. It's very enjoyable. Still, I don't spend all the time thinking about it - it's not some manga I'd write or read fanfics for, to quote an example. But still. Still. It's very, very good.

Overall
I don't deny that I think Switch doesn't fit the taste of many people. Why? Blood. Gore. Stories that are so realistic that could have happened with your neighbour, but still will give you nightmares. In fact, it's realistic enough to make people scared. There's the father who beats his child, the popular artist that does drugs, the group in school that does much more than bully the others, the student who has a relationship with his male and much older teacher.
Very creepy, but situations that actually may happen near you, and you just don't see it. That makes Switch is very worth reading.