The Sigma 50-200mm f/4-5.6 DC OS HSM is a very compact telephoto zoom lens with some nice features, including optical stabilisation, reasonably rapid and quiet HSM autofocus, and internal focusing. Even so, the stabiliser only consistently gave a 3-stop benefit in our tests, and handling can be a little awkward, since it's easy to foul the rotating focus ring during autofocus.
There's an impressive range of features packed into a very compact package.
The 50-200mm zoom range is relatively modest and sharpness isn't particularly good at the telephoto end.
Overall, the Sigma 50-200mm f/4-5.6 DC OS HSM is a decent buy at the price.