Sigma 50-150 f2.8 OS review in
I've had this lens for a while, and although I'm totally unqualified to give any technical appraisal, all I can add is that I have been very impressed by the quality of the pictures I've taken and the very low number of focus errors I've experienced. The OS works very well, and the pictures I took in a poorly-lit school hall (nativity play) were almost all sharp and perfectly focused (using an EOS 650D), even at f2.8.
I usually favour wide-angles, but this lens has given me an extra dimension. The size and weight came as a bit of a surprise, but apart from that, highly recommended.