What do you think of Tamron & Sigma Lenses coupled with a Nikon D80. Are they a good substitute for Nikkor lenses?

I ask this because, i'm on a budget. I know Nikkor lenses are more expensive than Tamron or Sigma lenses.

I want to get a general purpose lens with the D80 to start off my DSLR Kit.

Your help would be appreciated and valued.