Frankly, I didn't really enjoy GTO, and I'm not sure why everyone praises this manga to the skies. The first five chapters were horribly boring; kind of like reading a story about a loser who wasn't good at anything, and that's no fun at all. It was then entertaining and slightly addictive for some hundred chapters after I manged to plough through the beginning, but then it became... just... eh. I started to not care very much about any of the characters anymore, and it got awfully repetitive.
I really don't have anything against Onizuka - his near immortality, if nothing else, is funny - but he's the only interesting one of the bunch. Everyone else either supports/hates him blindly (if they're teachers), or loathes him and then ends up liking him (if they're students). Brilliant problem children also abound. And honestly, it seems that for a prestigious school, they have a remarkable lack of good teachers (who don't engage in peeping up skirts), and that the manga would have been better for a little teacher-sympathy. Was I the only one who kinda felt sorry for the principal? I mean, the guy was just pathetic enough to make you feel bad for laughing at him, and then he never catches a break throughout the whole manga. Overall, it had more of the feel of a delinquent school to me, not a prestigious private school.
Also, the art just didn't sit well with me. I'm not usually very picky with art; in fact, I like realistic art. However, GTO's art (especially when people are making faces), is actually kind of... creepy. I kept wanting to punch anyone who was being drawn with a pervy face. It's kinda crowded, too, and I had trouble telling the women apart. Generally art grows on me the longer I read, but I was seriously a hundred chapters in and it still rubbed me the wrong way.
Overall, I think it would've been a cool manga if it was shorter, but there are plenty of teacher mangas that were more entertaining than GTO. The plus points were that Onizuka had his cool moments and I rather liked the romance between the little nerdboy and the pretty girl. Other than that, I'm with Grumpy and Alphagulp (we could start a club! =D).