It's probably the best of Sakurai Michiko. It deals with themes that are very close to the heart, and in a way that is almost heartbreakingly realistic.
The main character is just... wonderfully deep. I love her flawed character, and the fact that she knows entirely that she's messed up, and why she's messed up. I love how when given the option, she tries to repair her circumstances, even though she just ends up getting hit in the face again with disappointment. And so, I completely understand when she attaches herself to someone that shows her affection, even if it's his job. Even if, when she learns a little bit about who HE really is, he jokes that he can fill the hole in her heart.
It's unbelievably sad because she isn't really all that innocent despite some of her reactions, and the teacher isn't either. It's sad because they're both caught up in their circumstances and mentality, and though all the characters keep on calling the MC a "kid," she's as adult as any of them (or at least grows to be), and that's precisely why she psychologically unstable - her loneliness basically sped up the growing up process.
And it is pure genius the way that Sakurai, the mangaka, makes an effort to say that people's problems shouldn't be a point of comparison - comparing the MC's issues, which are constantly referred to as trivial by adults, though they are devastatingly real and Taichi's supposedly "adult" issues is not the important part, nor should it be done. She basically says that it's the fact that those problems are THERE is the issue. That they are all messed up people in one way or another, and the depth of the problem does not measure the quickness or easiness of the solution. And that, if one DOES try to trivialize the problem, it can get (and oh yes it does in this manga) worse.
She's woven a story of people that need things they don't know they even need, want things they don't need, and lust after things they're not sure are even there. I loved her use of Takahashi as a foil character as well.
But this was hands down, one of the deepest heroines I've encountered. Taichi was a good character. I can't say I loved him, but he did his job, and his issues were interesting, to say the least. He was, after all, responsible for further tipping our heroine off into psychological instability, even if because of teasing/a joke, and he does bear the burden of his actions (or mistakes, depending on how you look at them).
That said, the way the mangaka tackles the characters' issues is the high point to this manga. The art style is not very typical, and frankly, lacks some finesse, but I grew to like it simply because it emphasized the raw nature to the entire thing. This is not coated in sugary shoujo melodrama--- it's a little more adult than that.
This manga is also probably one of the best host stories I've read, next to the Deep Love series (there's like 4 or 5 connected stories, each about 2 volumes each) written by Yoshi (or Yoshi Yuu).