Thanks, Thomas. I had the opportunity to try this lens at a large local camera store's demonstration days. The Sigma representative had a Canon 60D for customers to use, which, of course, would not let this full-frame ultra-wide lens show its full potential. I had hoped to return with 1D Mark II N and 5D cameras, but could not find the time.
I learned from another blog that this lens, advertised a having some level of moisture resistance, has no gasket at the mount, which, if accurate, would make this lens less desirable, as I had hoped to use it on weather-sealed cameras. Now, the new Canon EF 16-35/4L has become the leading UWA zoom contender for my Canon cameras, while the Nikkor 14-24/2.8G remains a potential choice for my Nikons.
Canon 5Ds R/7D2/7D/5D/40D/1D2N; Nikon F6/D3s/D700/FM3A/Coolpix A. Lens selection undergoing changes; some favorites: Zeiss 2/135 APO Sonnar, Canon 100mm f/2.8L Macro IS and 135L, Nikkor 14-24/2.8G and 200/2 VR.