I adore this manga. I think it's one of the justifications of reading StudentXTeacher shoujo in general. There's something incredibly pure about their love, and it somehow seems less ridiculous than how other series make it. It's not just "falling in love at first sight," but beginning to appreciate the other's personality and who they are.
This manga is basically about the flip side to the student-teacher relationship. In other shoujo, it's all about the "forbidden" aspect to the thing, but in this manga, it really focuses in on the student's burden, or rather what the student has to give up to date the teacher. "Sensei!" has plenty of character growth, most of which circles around the fact that in order to date an adult, you really have to be an adult yourself, or it's not really a relationship. (This fact is one that most shoujo manga tend to completely ignore, which makes the entire thing feel like a) pedophilia b) a big mistake or c) a guardian and his/her charge). That fact is also what I think makes this a masterpiece despite the sub-standard art in the beginning.
Hibiki grows into someone actually mature (with a personality, mind you), that the reader is bound to like, and Itou grows into someone more affectionate and willing to trust the ones he cares about. The side characters are not there for decoration, but propel the plot forward in their own ways, with their development astounding as well. I'm still not sure I could have stomached Chigusa as a main character, but she grew into someone almost likable.
One of the points I can't praise enough is the main character. She's not exceptionally smart, but she actually studies and (OMG!) earns good grades. Therefore, she is not the stereotypical stupid shoujo heroine. Though I have to admit, I could have jumped for joy when I found out that the plot has nothing to do with her failing some subject and having to go to extra lessons in which she falls in love with a teacher. She becomes gradually more mature, and though there was never a lot of crying in this manga, becomes someone strong enough to actually look up to.
There is much more I could say (even without spoiling the plot), but if you're willing to look past the art in the beginning, I think anyone could enjoy this manga. It's sweet, but not saccharine.