Quite a performer at a third of the price compared to the Nikkor 17-55mm f/2.8. The lens has the weight and a tight feel. Sigma's EX finish is also quite pleasant. However, their quality control needs some improvement. I have had/used my particular copy for over three years and although I have gotten quite a bit satisfactory photos, it is pretty obvious that there are some shortcomings.
The edges at f/2.8 is quite soft at all focal lengths but especially on the wide end. Stopped down would improve the performance. My particular copy has slight front focus that does not affect the photo too much but the right side is soft compared to the left which led to some fuzzy photos although focused correctly.
Even with the f/2.8, AF accuracy and speed is not good at low-light situations. The lens would go through the entire focal length range and hunts before failing to focus. Even if it did focus, it takes quite a few seconds that if the subject is a moving child, there goes the shot.
Overall it is a good lens to buy on a small budget but its performance leaves more to be desired. If budget is tight, I would suggest getting a faster prime such as the 35mm f/1.8G, f/2D; 50mm f/1.8G, f/1.4D for sharpness and speed. Zoom is useful, but I prefer sharper photos captured.