It's unlike most other shows in that it's morbid and highly visually stylized. Every character is completely insane (as in literally mentally ill) in a different way and it's still funny without total knowledge of the common references. While it has references, it's not like Family Guy, as it doesn't use unrelated vignettes and actually has competent writers who don't need to fill time up with over elongated unfunny jokes.
If you liked Pani Poni Dash, which now has newly released OVAs and is made by the same studio (SHAFT), you'll like Sayonara Zetsubou Sensei. In fact, I'd say they're basically the same thing with different characters and one has a darker tone.
The anime is better than the manga, but the plots are the same so if want to see if you'll like it or not just go to a manga-viewing site and look it up.
DESPAIR! YOUR JUMPING TO CONCLUSIONS ABOUT SHIT YOU HAVEN'T EVEN WATCHED HAS SENT ME INTO DESPAIR!
Oh, and teacher is far too focused on himself and his daily "problems" to get aroused by anything, much less his students who all want his dick.