Once again, I'll resort to an Italian expression in order to sum up a dorama, and in this case it's the old saying "Scherza coi fanti, ma lascia stare i santi" (lit. "Joke with the infantrymen, but leave the saints alone") which is used with the meaning that, whereas temporal things can be taken a little less seriously, sometimes, Faith should *never* be made fun of! (incidentally, as a teacher I've been taught - and wisely so! - that one should always thread lightly when discussing religion, so as not to insult anyone!)
And that's the first problem I had with this "Gomen ne Seishun!": I normally am a fan of Kudo Kankuro's style of hiding some deep messages amidst some silly, sometimes even outrageous, scenes...but this time I think he went too far and crossed too many red lines, constantly mocking both Buddhism and Catholicism as he did here (oh, btw - and before anyone jumps to the wrong conclusions - it's not like I felt offended because I'm Buddhist or Catholic - matter of fact, I'm neither; but then again, even though I'm no Muslim, I still think that Charlie Hebdo's "satire" is utterly *disgusting*!)
But anyway, even without considering all the above, the script was still waaaay below par: the main storyline about the ML's past and his sense of guilt was interesting and well done, but unfortunately, apart from the ML and a coupla more characters, everybody else was a mere caricature - unrealistic, idiotic and bidimensional. And every other storyline was poorly written, with characters constantly doing something in an episode and behaving as if nothing had happened in the next one...there was really no coherence nor consistency!
The cast did well, all things considered (or at least some of them did, namely the actors Nishikido Ryo and Yamoto Yuma, and the actresses Moririn - no surprise there, LOL - and Kuroshima Yuina - she totally *charmed* me! <3), and the OST was kinda catchy, but the script's many faults made it so that this can't get the passing grade, unfortunately.