i like this much better than GTO. i watched the anime because it has a couple of my favourite seiyuu, and that intrigued me enough to want to read the manga. i'm glad i did, because it's a gem. don't let the art turn you off; it's pretty bad at the start, but improves considerably over time. also, if you come from shoujo, the character design might turn you off -- the heroine isn't beautiful, and many of the characters are in ugly-boy territory. i didn't care, because bishounen who all look alike are a dime a dozen -- one has no trouble at all to tell these characters here apart.
but best of all is yamaguchi, the protagonist. she's competent, passionate, honest, loyal, and caring. after so many simpering shoujo protags, it is such a breath of fresh air to meet a female character who can handle herself, who doesn't need rescuing (who is, in fact, the one rescuing others), who cares about something other than romance. and yet she's also not perfect; she makes mistakes, and she can admit to them, apologize, and make amends. i love that she doesn't give up on people even if they really screw up. if only we all could have such teachers.
while the manga isn't about romance, there is romance, and i think it's handled very interestingly. for most of the manga we get to see the romance only from sawada's side, because yankumi doesn't think of him that way, but she grows closer to him because he always watches her and comes to understand her better than most. i really like him as a character; he's so bored with everything at the start, acts so aloof, that it's fun to see him start to take an interest and become more and more involved in life. the people who say he is bland are not looking closely enough -- he controls himself tightly, he doesn't let anyone see him sweat (no surprise considering his father), but he cares about his friends, and is always there to get them out of serious trouble. at the same time he is a dorky teenager, who still gets into trouble himself. it's fun to see him grow more and more mature, and become yankumi's confidant and partner (in a non-romantic sense); a force for good. i don't usually ship people, especially not het, but i am a total fan of "shinkumi" because of the slow development that feels very real to me -- and that in the midst of yakuza, over-the-top fights, and non-stop comedy. it's very deftly done. so, i highly recommend the manga -- it's better than the first live action (and let's not talk about the 2nd and 3rd, which are yawners -- now that i've read the manga i really wish they had stuck with that instead of doing their own thing).