I have been looking at some possible Sigma lenses, which would nicely fit to my Nikon D80. So far I only use super zooms from Nikon (18-200mm VR and 70-300mm VR), which unfortunately have not very low starting aperture. I also use a fix 50mm f/1.8 lens from Nikon.
The following Sigma lenses caught my eye:
Sigma 24-70mm, f/2.8 EX DG Asp. Makro, Nikon
at Digitec for CHF 550.--
or the newer "updated" Sigma 24-70mm, f/2.8 EX DG HSM, Nikon
at fotovideoplus.ch for CHF 659.-- (not yet avail.)
Sigma 70-200mm, f/2.8 APO EX DG HSM II, Nikon
at Digitec for CHF 947.--
What I like about the Sigma lenses is that they are about one-third to half of the price of a corresponding Nikon lens, but according to some reviews still seem pretty valuable alternatives to the original Nikon ones. Personally, I can do fine without the Macro functionality, I rather would want to have a fast auto focus, therefore the HSM on the new updated 24-70mm is a plus. Although it might be slightly more expensive and my favorite so far, it is not yet available. The 70-200mm is kind of a no-brainer. So I guess I will have to start saving for those lenses or should try to get rid of my old Nikon super zooms. Although my Nikon 18-200mm is a very handy and useful lens, I'm not too happy about the quality of the photos (regarding sharpness and resolution). I now made some shots in Egypt with low ISO values and the VR (vibration reduction) turned off, but I still need to evaluate those regarding improved quality compared to using the lens with VR turned on.
Here are some reviews of the mentioned Sigma lenses:
70-200mm: dpreview, FM Reviews
old 24-70mm with Macro: the-digital-picture, FM Reviews
For the new 24-70mm HSM I found no review yet, but a news post on dpreview and some discussions.