Sayonara Zetsubou Sensei is a pure sketch comedy set in high school, even more so than anime such as Azumanga Daioh. There is no plot, no character development, and humor can be random and nonsensical at times.
ART - 10/10
This anime has very good art. The animation and music are both interesting and unique. This is certainly it's best trait. If nothing else, this anime is a visual and auditory experience.
CHARACTERS - 5/10
A collection of characters are introduced in the first few episodes, and they all seem interesting at first. However, these characters are entirely one dimensional. Now for a pure comedy, character development usually isn't a requirement. The fact that many jokes use the character's traits as the punch line, though, means that jokes are effectively repeated - the setup may be different, but how much variety can there be when it always results in the same punch line?
OVERALL - 6/10 - Decent
Sayonara Zestubou Sensei has a great artistic direction, and it's jokes are funny - at first anyways. Once all the characters are introduced, the jokes based on the characters start being recycled. Luckily, there is still plenty of comedy that doesn't rely of the characters for the punch line, but the stale, repetitious jokes certainly take some of the bite of this anime. It can basically be seen as a much better Doki Doki School Hours, although still suffering from the same problems that one did.