Aku no hana has an somewhat interesting story setting, but nothing original or something out of the ordinary.

The protagonist steals the gym cloths from the girl he likes and is seen by a quiet girl in class.
He's the typical spineless, crying guy, who is getting blackmailed by this evil mysterious girl in his class to do all sorts of stuff for her, just to prove that he is a real pervert, while he continuously tries to prove that he actually is innocent and a good guy.

So the story seems really predictable this way, that this mysterious girl and the protagonist will end up together instead, or that he will confess his evil doing to the girl he likes and who then will get hurt, but forgive him eventually.

But this is not the case, oh no.. not at all. This girl is beyond ordinary, everytime you think that the climax has been reached, she pulls you deeper into the abyss of discomfort and agony, she tries to bring out the protagonists real self, which is reflected in the dark books he always reads.
Many manga's try to portray insanity, but often fail to do it in a way you can understand them and feel with them, this manga however i believe, has achieved this.
It has a scene where you really felt it build up, the insanity, where reason and logic didn't matter, a quite powerful scene.

So if you ask me if the plot is well written? I would properly have to answer, not really. If the art is good?It's actually quite poor. Character designs? They're stereotypes slightly different, but become something different, so they're somewhat interesting.
Did i enjoy it?Oh yes, i certainly did, i sat on the edge of the chair sometimes, it really entertained me.
So if you look at every little aspect logically and try to break it down, this manga is nothing special, yet, somehow, becomes very entertaining as a whole.

(read until chapter 13)