I own a Sigma 50-150 mm F2.8 DC HSM. It's a great lens sepcially for travel. My Canon 70-200 mm L USM is very heavy. I also own a Sigma 24-70 mm f2.8. I have some focusing problems at 70 mm wide open at 2.8 using autofoucs. I use a Canon 20D and a Canon 400D
This is my absolute favourite lens out of the SIGMA range for my SD14 !
AF is fast, silent and the overall lenght of the lens doesn`t change at all
no matter if close focus or infintiy distance is set.
On some occasions the 50mm is still a bit to longlensy...here the 24-70mm would be the right support lens.
I use the 50-150/2.8 HSM II. I like it although it has a weak point at 50mm. Stopping down to F4 helps at any range to improve the image quality. As others said, the major advantage is the weight for an F2.8 telezoom.
I would not mind if Sigma would come out with a simlar lens up 200mm and F4 as the widest aperture. For the same weight, we could get the an even better image quality.
But the new Nikon Zoom 70-300(4.5-5.6 VR is compared to the Sigma 50-150/2.8 HSMII IMHO slightly better. But it is not fair to compare both, since the Nikon telezoom has worse aperture and same prica tag. So Nikon have more margin to offer better image quality with that telezoom.