Just like the last Sigma (see 4/6/11 post) lens reviewed, I used three lenses for this review, and all three seemed about the same in image quality, so I’m thinking the quality control for this lens is ok.
The Sigma 70-200mm OS build quality is good, (but not as good as the Sony), and it offers the same type of fast, almost silent focusing system that Sony uses (SSM). Unfortunately, there are a couple of things missing on the lens that should be included in this price range; like some focus hold buttons and a focus limiter option to speed up focusing in certain situations.
The Sigma 70-200mm F/2.8 OS is pretty sharp from F/4 and up near the short end, however, as you zoom out near 200mm, the image sides are not as sharp as the Sony or Tamron equivalents. There are a few impressive qualities to this lens, like the stunningly sharp centers at F/2.8, 200mm, and the very smooth out of focus highlight blur (bokeh). The lens based optical stabilization works well, and offers a little more practical advantage than Sony’s Steadyshot, see that comparison here.
Overall, I think the Sigma 70-200mm F/2.8 HSM telephoto zoom lens with optical stabilization is not all that impressive considering the price tag is in the neighborhood of the excellent Sony 70-200mm F/2.8 G.