Good day class, now today we are going to learn how to make one's self taller, now excuse me while I pass out the nooses...

STORY: Sayonara Zetsubou-Sensei is a dark satire comedy manga by Koji Kumeta, and yes it's the kind of dark with notions on suicide and the gross underside of society (With a satirical pinch of looking at the light side of the underbelly of things).
Each and every chapter are mostly stand alone stories, so there's not exactly a real story to talk about, but rather they're like school lessons, each chapter goes into a certain topic and solely focuses on so. From topics ranging from culture to hibernation, from mistakes that people make to why college students who are studying for exams get certain privileges and so on, but not in a normal sense, but from how Zetsubou-sensei himself perceives.

Zetsubou-sensei (Literally meaning Mr. Despair), or his actual name, Itoshiki Nozomu, is the main character of the series and a teacher of a predominant female class in a middle school who looks at everything negatively and wants nothing more than to die by his own hands (You'll see him try to commit suicide on a constant basis).
Now when I said he's a teacher, the truth is he doesn't exactly teach his class per-say, but instead rants about something, saying nothing but the bad about it to the class and then after he's done he goes into despair as he gives up hope on whatever he was ranting about. So then after hearing his single minded preachings, several class members decide that's enough as they go on to counter argue what their sensei had said, trying to convince him that not everything about so-and-so is not all that bad as he wants to believe.
That is the usual routine of each chapter, with each chapter ending with ridiculously absurd and bizarre conclusions that you might even learn something from.

Aside from sensei, the class itself is full of the most unique and hilariously quirky characters, though they all have just one single character trait going for them, they would not be anything anyone would call average, typical. or normal (Except one of them, that's what the character's trait is, being the normal one, and is even insulted for being normal).
These characters aren't your typical tropes like tsundere or yandere, but instead how about a girl that only sees everything positively (The literal exact opposite of Zetsubou-sensei) or a girl who can only communicate by sending text messages, nasty insulting text messages. Instead of your typical shy girl, why not go a couple of steps further to a Hikikomori (A shut in) girl.
And with the episodic formula of the manga, expect to see each of them featured in their own chapters from time to time.

Even with all the hilarious and absurd characters, the great satire and dark comedy, and the simple yet effective use of it's story in episodic manner, there is one very pressing issue with Sayonara Zetsubou-sensei: The majority of the references and jokes are very, very Japanese.
By Japanese I mean do you know who Hiro Tsunoda is? How about Hitonari Tsuji? And honestly if you do not even know what a hikikomori or what a neet is, then this will not be very funny towards non-japanese manga readers. And these references are not one in a few chapters, they're once in a few pages in every chapter common, but if you consider yourself to be very versed in japanese culture, then there shouldn't be a problem.
If not, then get yourself a handy dandy japanese reference notebook or prepare to wiki everything. (But for those who want to read from the english language releases of this manga, there is an impressive guide for just about everything referenced in the backs of the manga volumes).

ART: Now to be fair, you shouldn't be reading Zetsubou-sensei for the artwork. It's not bad or anything, just don't go in expecting the artwork to be equal to the comedy.
And no it's not bad or ugly at all (Though it is intentionality ugly for humor at times), it's just simple, but it gets the job done (And the art is good enough to show you the secret of the universe).
The character designs are diverse enough, but the female characters do look all the same just with different hair styles, but not to the point where you can't tell who from who.
But this a definitely a not manga you read for the artwork.

OVERALL:
The Good:
+ A hilarious, strange, and insightful dark satire comedy.
+ The characters are unique.
+ Simple and effective use of episodic story telling.

The Bad:
- Very japanese, not exactly funny for people not versed in Japanese culture.
- Simple artwork.

You can still get plenty of laugh from this manga, despite not knowing japanese culture, because there's plenty of absurd comedy to please that funny bone.
But hey if you're not knowledgeable in japanese culture but you want to read a comedy manga anyway, I suggest you go pick a shonen jump and read Gintama, just make sure you know what a shonen jump is.