Sigma have always been fairly canny at plugging holes in other manufacturers ranges, lenses like the 12-24 (full frame) and the 50-500 bigma were at the time of their launch pretty unique to Sigma.
I've owned a fair few sigmas over the years, the DC 10-20 f4-5.6, the DG 12-24 mk1, the DC 18-50 f2.8 EX, the 24mm f2.8 MD II, the 28mm f1.8 MD (one of my most favourite lenses ever) DG EX 70mm f2.8, the DG EX 50-500 and the 600mm CAT (on Minolta MD and Canon EOS) and I would vouch for all of these, I still own and use the 18-50mm and the 70mm, both of which were best in category at time of purchase.
I've owned 2 sigma lenses that I wasn't happy with, the 17-35 f2.8-4. I kept it one day and returned it and got a 17-40 f4L instead. It was a horrid horrid lens, and I had the 30mm f1.4 DC, which I loved for video (in MF mode) but was terrible at focusing, +18AFMA on my 7D and unusable on my 600D. Also returned.
I've not had any hands on with their new, well reviewed art series lenses, they seem to be getting very good, but also getting less of a value proposition. Other manufacturers also seem to be plugging interesting holes, like Tokina with my 11-16 f2.8 UWA. Nobody else makes anything like it.
I am tempted to get the Sigma 8-16mm, but theres a few more practical things on the list ahead of it.