I have two Sigmas:
The 28-105 is ok and a great normal zoom for your first SLR or as a spare lens. But it is not a suberb performer, but I have taken some great photos with it (see my gallery).
I am about to trade it in for a Sigma 28 mm f/1.8 and a Canon 85 mm f/1.8 (I have Canon 50 mm already som these three will do for me in these focal lenghts).
The 70-300 is a very great tele zoom lens, and the APO makes the quality high with superb sharpness.