Note, this is version II of this lens.
After 2 trips to Sigma for service (they are very fast and easy to deal with) this lens no longer front focuses. It does seem to have some issues with accuracy near MFD at 150 though, but I would probably manually focus in these circumstances anyways, and it is rarely used this way. The combination of f/2.8 in such a useful range at this size/weight is nothing short of wonderful. I can get images that rival the 70-200 f/2.8 in a lens that is much smaller and much lighter. It has been good for both wedding/event work and sports. Autofocus is fast, contrast is good when not shooting into the sun, and color is sigma but very excellent. In addition, I find the bokeh more than acceptable. The size of the zoom and focus rings is also very large, which makes use a joy.
All in all, I am very happy with this lens. If it had IS, it would be better. I will probably end up purchasing the 70-200 2.8 IS at some point, but may end up keeping this lens, just because of it's small size and great focal length.