What struck me with several lenses from Sigma, Vivitar (some time ago) and Tamron was that the center was pretty good, the sharpness dropped noticably in the corners and vignetting became a problem as well. Since you generally use a digital, which does not use as much of the outside of the glass, perhaps you can avoid that part of the lens.
By the way, that Nikon lens you mentioned has IS built-in, and a recent review I read about it said that it automatically adjusts between normal IS mode (vert. & Horiz.) to panning mode (vert. only), which can be quite useful for us.
I guess there are some exceptions. That Sigma lens of yours certainly has seen a lot of airplanes (and obvoiusly gone through a lot of chain-link fences

). I know another guy at DFW who swears at (sorry, by

) the same lens.
See ya,
Charles