I watched this show growing up and it did not affect me in a negative way.
A lot of stuff I didn't even get when I was little--the sexual references, the one about the guy they think is a cokehead. I still loved the jokes though, and the delivery is great. I'm sure not every kid will like it, but it's really not a bad influence. This show is mild compared to some kids' shows today.
The characters are so ridiculous that a child could not possibly think they are role models.
They also show a side of everyone, a very human side that is neurotic, selfish, obsessive, and cynical. I think it's important that kids realize that this is a part of life, though.
And even if the main characters aren't good role models, the people they meet aren't usually either--George's boss that he poisons later, the drunk who leaves Jerry to take care of his dog, the yogurt store owner that pretended his yogurt was fat-free...
There are people like this everywhere, and kids need to know that not everyone is decent, and everyone has their flaws.