Conclusion: Agai, lack of Sigma's QC: i bought one second hand, but this lens is a lemon, you can throw it away. The lens was soft overall no matter what i used, really bad till f7.1, from f8 till f22 was overall soft. I think HSM doesn't exists in this lens because the 15-30EX without HSM was faster than this copy of mine. What i can say if you don't need a full frame lens you better buy the Sigma 10-20DC lens...
Cons: lack of Sigma's QC, lens is soft overall, slow AF
Summary: To be frank, I think this is a toy lens. It doesn't focus precisely. It produces blurry photos always even if you use a tripod. It has just a wide view for nothing. The angle is pretty wide, but too soft focusing to use as a pro lens. In addition to that, although it might have quite a wide view, 20% of the picture area is useless. It has vignettes on the periphery of the frame. AND also, it doesn't focus well.
Summary: This lens did not work well with a Canon 5D Mark 2. Horrible distortion along the edges... kinda "slow" at f4.5, and it totally failed to work (ie, auto focus mechanism) in -13 degrees (I live in Anchorage, AK). None of my Canon lenses failed to work at much lower temperatures. In fact, when this lens consistently failed to work over several days, I put on a Canon lens in the identical environment and I shot for 30 minutes with no problems.
Summary: This is a fun, playful lens. I wanted to use it for architecture. At 12mm is was vignetting my camera chip by 20% and at 24mm with full coverage it was not doing what I needed. It created soft images compared to the more expensive lens, in addition to its usefulness in rendering structures. For my application it was the wrong lens.
Summary: I was attracted to the ultra wide zoom range for my full-frame Canon camera. The view at 12mm is truly remarkable. Compared the Sigma's IQ to my Canon 16-35 at f/8 at 16mm and 24mm. Unfortunately, center sharpness was visibly inferior to the Canon, and the Sigma's edge sharpness was extremely poor. At the Sigma's 12mm setting, there was significant linear distortion at the edges of the images. I ended up returning the lens.