After intensiv testing Canon's and Sigma's fish - in diffrent light situations - I went for the Sigma, as it showed better results on a 10 D. CA was bigger on the Canons fish, the Sigma has it too, but less expressed. CA depends much on light situation, but I'm not really able to tell when CA will occure. As this is my first and only Sigma lens - and I will not go for other ones, I was positivly surprised. Even after defishing, some 30/20 cm/300dpi files look good.
My others: 17-40, 50 mm macro, 100 mm macro.