
After painfully doing a lot of "homework" and speaking to some who have some great experience(thanks Dick M.) on various telephoto lenses for DSLRs including all of the "third party" glasses like Tamron, Sigma and Tokina, as well as most of the Nikons I have made my mind up and will soon be getting the Sigma APO 70-300mm for my D80.
Shooting mostly with the Nikkor 18-135mm f/3.5 - 5.6 kit that came with my camera and I am very happy with it for the most part excluding with the wider end (18mm) in which it does get a little "dirty" but again, it does a great job for a kit.
Dick McGowan also shoots some with a Sigma and you can see some of his pictures here: http://www.flickr.com/photos/stackedplates/
He has taken some incredible pictures including some great stormscape shots like the one below of the Hill City "Mothership" also shot with a Sigma lens!

Copyright Dick McGowan (please contact him for copyright usage at: http://www.tornadolive.com/)
For the price and quality, (as you can see from above) Sigma seems to have great products that produce great results. This is why I am going to choose the Sigma brand in the future.
I plan on posting some new pics as soon as I can which include a few HDR shots.