I had watched the GTO movie and that led me to watch the GTO drama. Its an older drama so you have to give it some leeway for that . . . 1998 is different then now. Really.
Anyway its the story of a man with no qualifications that wanted to become a teacher despite having a shady past. This drama shows how his unusual ways help his students to become better people and achieve their dreams. This can be a bit repetitive, but is better then some about story originality.
Onizuka is a great character, though I often did worry he would do the wrong thing . . . which actually speaks to how well the drama is written and to what a great actor Sorimachi Takashi is. He really makes the show and is the main reason I suggest you watch this movie. Later dramas dealing with shady teachers really draw off of this one, so if you've like things like Gokusen then you really need to watch this.
I would highly recommend this drama for people who love high school dramas.