Optical quality is good and the 4-stop optical stabiliser performs well, but it's a shame that Sigma didn't fit an HyperSonic Motor (HSM) autofocus system to the Sigma 70-300mm f/4-5.6 DG OS. As it is, the basic electric motor is quite noisy, but at least it's not too sluggish in operation.
The 4-stop optical stabiliser makes for consistently sharp handheld shots, and the build quality feels robust.
It lacks HSM autofocus and the front element rotates and extends during focusing.
Overall, the Sigma 70-300mm f/4-5.6 DG OS is a good buy at the price, but it's still outclassed by the more expensive Nikon 70-300mm VR and Tamron 70-300mm VC USD lenses.