The Sigma AF 50-150mm f/2.8 EX DC OS HSM is an impressive lens, both in terms of performance, but also size-wise.
Image resolution is very high across the frame. Vignetting is a bit pronounced wide open at the longest focal length, but otherwise no issue. Distortion is well controlled and CAs as well as bokeh fringing are very low. The lens delivers very smooth bokeh at large apertures (less so when stopped down).
The build quality of the lens is excellent. Thanks to a HSM drive the autofocus operates very fast and silent. The optical stabilizer allows for up to 3 stops longer shutter speeds in the field and subjectively performs in the same level as Canon's IS and Nikon's VR II implementations. Unfortunately, the lens is quite large, so there is no size advantage that is typical for many other dedicated crop lenses.
So, who is this lens for? Obviously, the Sigma competes with native FX lenses, the high-end 70-200mm f/2.8 zooms. On DX cameras, it offers less magnification at its longest focal length, but adds flexibility at the short end of the zoom range. There are applications, like weddings and concerts, where this makes the lens more attractive to existing DX shooters.
However, one should also be aware of the fact that the Sigma resembles the field of view and depth of field of a 70-200mm f/4 lens on a full frame camera. A 70-200mm f/4 lens on a FX camera is not neccessarily cheaper (especially if one takes the camera into account, too), but offers a less bulky and most likely lighter package.
Still, to those who fit the narrow niche the Sigma aims at, it is highly recommended.