I'm not a huge fan of Invader Zim... the first episode was complete ****, but the rest are OKAY, I guess. There are some moments in that cartoon which are doubled-over laughing moments, but those are few and far between. For the most part, I find the show repetitive and irritating, much like the kids who watch it. The majority of the audience for that show seems to be kids who think they are dark and mysterious/vampires/emo, but there are, of course, exceptions to the rule.
It's a show that tries too hard to be funny, and as a result of that most of the humor just comes out awkward.
That's my two-cents, but I watch SpongeBob, so what do I know? Now, I just gotta figure out how to **** myself, and I should be okay.