The kit lenses work no doubt, but I just do not like them.
People say it takes great pics, but until you take the same picture with 2 lenses side by side, you won't really know how bad they are and what you are really missing.
I had a chance to take a few pics with the Nikkor 18-55 kit lens and it was sadly... an absolute joke. Compared to the pics taken with my Sigma 18-50 side by side, the Nikkor lens looked faded, lightly blurred (it was NOT out of focus!) and the colours were not crisp. If you zoomed the pic on the monitor, the situation became even worse. I used it once becuase it was a loaner while my 18-200 was getting checked out and I had no desire to use it a second time. Yes, to me it was that bad.
The Sigma is more expensive, but dammit, you have a D80, a near thousand dollar camera... why put a lens on it that will result in pictures who's quality will be about the same as a $200 point and shoot camera?
Kit lenses are for people who are extreme beginners that either do not know better or do not care about the results of their pics.
Once you slap on a good lens on your camera and see the effects, you will honestly feel sorry about spending that $300 on a cheap lens. That money could have been placed towards a proper lens that gives you way better results and will serve you well for years.
As a general example, I do not consider the 18-200 a kit lens, but had I known about the 70-200VR, I would have had that in my bag instead. As it is, I will be ordering my version of it likely in January after the Christmas rush and spending spree. The money that I invested in the 18-200, well thats basically money lost, even though the 18-200 is a very good lens, its not a lens really worthy of my D200's capabilities when compared side by side.
It's all about doing your homework... reading articles, reading reviews on the net, talking to people that have it, looking at pictures taken with that lens critically and doing lots of shopping around for a fair price and good service.