This has got to be my hardest working lens ever! Now about 5 yrs old, I have had more than my moneys worth out of it. Superb overall sharpness at all apertures and focal lengths. Absolutely rectilinear characteristics make it superb for interiors. (Just remember to keep it level unless you want converging verticals!)
What I thought would be its biggest drawback - the exposed "popeye" front element has proved to be no problem at all. I remove the lens cap when I want to shoot and replace it. Occasional light wipe with a lens cloth keeps it pristine.
This has got to be one of Sigma's all time classic lenses.
Only drawback of any significance is its bulk and weight in your camera bag, but this adds to it "quality" feel too, so there you are.
(Written by: kiwijohn)