Invader Zim is very worth watching. Zim is an alien (Irken) who is intent upon conquering the Earth, which he believes he's been assigned to do by his home planet. Actually, the people in charge of the alien planet just sent him to Earth, which is to them a piddly nowhere little planet, to get him out of the way because he is such a screw-up.
Zim is completely unaware of the fact that everyone regards him as a screw-up, and is very fanatical about his quest to conquer Earth.
Zim has a goofy sidekick robot named Gir who was taken out of the garbage and given to him when they sent him on his "quest" to conquer Earth. Gir is a hilarious character, and is part of what makes theseries so great (he reminds me very much of my cat, I think I would've named her Gir if I had seen any episodes of Zim before I got her).
There is one person in the whole universe besides Zim who actually takes his quest to conquer Earth seriously, and that is his nemesis, Dib. Dib is a human boy who is obsessed with aliens, and he is the only person who realizes Zim is an alien. Dib tries to tell everyone that Zim is an alien but no one believes him, so he singlehandedly tries to thwart all of Zim's plots. It always backfires on him, and Zim's plots always fail anyway.
The artist who wrote and drew the series is Jhonen Vasquez, who was known before Invader Zim, for the "Johnny the Homicidal Maniac" (JTHM) and "Squee" comic book series. Invader Zim is more than just a comedy, the artwork is great and every episode is full of Vasquez's characteristic dark, subtle mockery of all the stupid things about society.
I think Invader Zim was excellently done and deserved a longer life than the less-than-a-year it was given on Nickelodeon. I believe this was because it was being shown in the wrong venue. I just don't think that the people who turned on Nickelodeon on a Friday night were expecting or looking for anything like Vasquez's dark, intelligent humor. I think that most people who glanced at it just assumed it was more of the same-old meaningless Nick-toons, and never really ever tried to watch it(not that I mind Nick-toons, but not even "Rocket Power" or "SpongeBob Squarepants" come close). I myself might never have sat down to watch Invader Zim, except that, being familiar with JTHM and Squee, I recognized Vasquez's artwork when I happened to catch a commercial for the show.