Sayonara Zetsubou Sensei is the first comedy manga that I've read. While the overall story that encompasses each chapter isn't that important (a teacher that wishes for death has to teach a class with the most optimistic girl on earth), the subject seems to change from chapter to chapter. While some of the humor is hard to understand for those that aren't Japanese-born, there are helpful explanations at the end of each volume, so I laugh a lot at myself for not understanding some of the humor. While I've just started with this manga, I'm going to continue reading. I suggest you check it out the next time you need a good laugh or two.
Story - 10/10 I enjoy that each chapter is its own experience, so you don't necessarily have to read every single chapter to get the full story.
Art - 8/10 The art isn't the greatest that I've seen, but that's how the series was drawn. Art really doesn't make or break a manga for me.
Characters - 9/10 While some of the characters are your average "harem" of high schoolers (foreign girl that plans to sue everyone for everything, girl that is an internet nerd, optimistic girl, tail fetish girl), they are still very memorable and lovable.
Overall - 9/10 This manga definitely deserves a read in my book. At least read a few chapters of it to see if you really enjoy it. This series definitely deserves a lot more attention than it has received.
The story changes depending on the subject matter of each chapter but the characters create a witty and funny experience for the individual subject. If you enjoy laughing at other peoples' and societies obvious mistakes presented in dry humor, you are sure to enjoy this manga.