Before getting my D70 a couple of weeks ago, i was using a Nikon F90X with just one lens - a Tamron 28-200.
I got the 18-70 kit lens with my D70 and just use the Tamron when i want the extra telephoto. This has shown me that there is a world of difference between the 'feel' of the two lenses, especially the speed of autofocus. I haven't yet had the opportunity to test and compare their optical performance but i fully expect the kit lens to knock spots off the Tamron because the pics that i have taken so far seem very clear and sharp.
I know nothing about those Sigma lenses and i do know that many Sigma lenses get great reviews, but I also know that the 18-70 kit lens is a steal if you buy it as part of the kit.
If the 18-70 lens did not suit my requirements, i would even consider buying the kit with that lens, selling the lens (for more than you have paid for it as part of the kit) and then getting lenses to suit my needs.
The kit lens does suit my needs at the moment and i think that any future lenses that i get will also be Nikon AF-S lenses coz this one works so well