First of all, if you're not used to reading profound psychological works, don't read this. Read some other things first to get some experience under your belt. If you don't understand what a gem this manga is (whether or not you love it), then it's worth is lost on you, and you should read it later.
This manga breeches on basically everything. It leaves no stone unturned, which is a daring task for anyone. It questions humanity, "what is a human?" and the ability the environment has on the individual, "what is the relationship between society and the individual" among numerous other things. Another thing is that there are no gaps in this manga. It may seem very "thriller-like," but it is far deeper than that. It is not a romance, nor is it romantic. In fact, it would not be a stretch to say that this manga mocks the romantic view of life.
The characterization is just wonderful, and the struggles the characters go through unearth startling, possibly valid conclusions about the meaning of life and humanity.
Note: This manga is highly disturbing as well. If you are completely grounded in your own views, don't bother wasting your time even trying this–– you won't understand it. This manga requires a looser mind to really understand even half of what "really" goes on in it. It goes against the basis of religion, the overarching meaning of life, and even the idea of a natal conscience.
That said, if you feel like you can handle it, read it. I do not exaggerate the amount of depth inside it, nor the requirements to truly understand it.