I am one of the people that definitely agrees with Sigma's lenses being good quality. I have owned or used the: 105mm EX macro, 50mm EX macro, 24-60 f2.8 EX, 28-70 f2.8 EX, 80-400 EX OS, and 70-300 DL Macro Super.
The only lens I have been disappointed with was the non-EX. Sigma makes some really awesome gear, and most times at quite a price difference between them and the brand names.
As I will always caution anyone looking for equipment... Try it first if you can.
Try not to get too caught up in the gear... It is the end result that matters most!