Good lense for the price. For the range up to 300mm, this is an affordable lense with acceptable quality if not fantastic to some.
Only limitation is at 300mm, if low light, even high ISO susceptible to "handshake". So that is why the usefulness of IS or OS. Cost a lot more. Of cos, if you can put in the extra $$$.
Another is the AF, but still ok for me as it is improved over the old non-digital version, quicker focus & less hunting, just focus on contrast area near the subject if not use manual.
understand & use within the limitation.
If no extra $$$ for the IS/OS, USM FTM, L lense, etc ,etc, go for this sigma & use in "bright" lighting condition, still give u good pics.