I received my Sigma 17-50 2.8 Ex today.
I've only had less than half an hour to play with it but so far it seems to be excellent. Very quiet and quick AF, Nice build quality. I haven't had a descent look at the shots yet but what I could tell from the 550's screen they were nice and sharp,the colour was very nice as-well(different to the kit lenses in a good way) and the bokeh looked to be quite nice also. I just have to remember to keep the fingers clear of the focus ring during focusing. The unit came supplied with a nice wee baggy and petal type hood.
Once I've finished my current assignment(which I should be doing now) Ill post a few images and some further thoughts.
So far though Im very happy!!