I have the Sigma 17-35mm EX DG, which I think was a second attempt at this zoom. According to Fred Miranda, it was thought little better than the Aspherical.
Mine was tested back-to-back with a Nikon 17-55mm at a shop, and I thought the Nikon was better, but only wide open and near the edges. So mine is OK. I think copy variation is the issue here.
If you are on a crop body, it will likely seem better.