As a reader who absolutely adores josei/shoujo as long as it's done well, this is probably one of my favorite josei manga I've read. Ever. It's honestly purely hilarious and full of life and energy. Plus, one of my favorite characters ever is the female lead! Yamaguchi is so great. Though she's painfully unaware of everything to a point, she somehow is also extraordinarily cool and mature. She's very well-fleshed out as well, and though her intentions at certain points are mysterious, she still makes for a really awesome character. I wouldn't say that there was a huge amount of character development, but it is pretty obvious that she changes into a more mature role while also being the funny teacher/yakuza that she is. This is truly a strong female-lead-driven manga, and that's probably why I adore it so much.
Besides the fact that she's an absolute BAMF and awesome, I actually really liked the male lead in this one. Shin is really cool, but he is a bit of the stereotypical "cool" guy, and I don't know, I think it would have been cooler if he was at least a little bit insecure about the fact that Yamaguchi was older, because wouldn't anyone be?
He does get a little insecure at one point when another "love interest" is introduced as the "second guy", but it's pretty clear that she would end up with him in the end.
Even though I really don't approve of teacher-student relationships in manga, this was done somewhat well. However, I felt that the romantic development was really very slow, despite the fact that you could tell that Shin liked her from the start. The author pulled one of those "romance only happens at the very end" moves that I'm not a fan of, but this manga isn't really focused on romance, per se.
I think one of the selling points of this manga is more about its storytelling/plot development. It's addictive and absolutely refreshing and original. It doesn't focus too much on romance, but that's exactly what makes it so unique in comparison to other josei and shoujo. Though it is a bit unrealistic, I really adored the yakuza portions of the manga, because it was so interesting! Yamaguchi's second life was well-explored, and the people in her "other life" were some of the most heartwarming, wholesome, and hilarious characters I've read in a while.
Finally, I've seen a couple of reviewers complain a little bit about the art. Every manga reader has their preferences, and I know that 90's art isn't usually well-loved, but I thought it was nice, simplistic. It wasn't too hard to see anything or distinguish things. And I mean this as a reader that finds it very hard to read manga with bad art, like the beginning of Basara. Perhaps it is a little different from the whole "beautiful" and "clean" shoujo scene, but I felt that the art really did fit the storytelling. Yamaguchi and the other characters aren't supposed to be super beautiful and such, but that's the point.
TL;DR: Overall, I felt that the romantic development was much too sluggish. On that note, this manga isn't necessarily focused on that, and that's what makes it so different and interesting. It's a character-focused and plot-focused manga with convoluted arcs and some of my favorite characters ever. The female lead is not only strong emotionally and physically, but she also serves for some of the best comedy relief. Therefore, 9/10.