I do own a Sigma 50mm f/1.4 though and I have been very happy with it...That lens has been on sale lately for under $400 and is a really good alternative. I would be interested in seeing it upgraded in the Art Line, tho... It is great to see Sigma improving the products and their service..I like lower-cost alternatives but I LOVE lower-cost leaders-of-the-pack!
On the other hand....after reading some of the compatibility issues on Sigma products on evolutionary upgraded Canon camera bodies..it would give me some pause on spending any "real" money on a Sigma lens without a guarantee of firmware update capabilities for new Canon bodies. The company seems to be addressing this issue with the lens dock....but their past performance with that issue is unsettling.