Sigma really came through with this lens. If you have a 1.6x SLR, this the perfect walkaround lens! Because it doesn't waste glass focusing a full-size 35mm image, you get amazing performance in a relatively small package.
CA is very well controlled. Purple fringing is only noticeable on RAW images at 100% -- and that's with power lines and branches in front of a clear, sunny sky. Picture quality is at least as good as my 200mm 2.8L series II.
Overall, I'd say the images look better than the Canon 17-40mm 4.0L -- not surprising, since the 17-40 is built for full-frame 35mm. Long story shot: good job Sigma, this is my new favorite lens!
Bought from sigma4less.com. They were fast and didn't try to rip me off or upsell me. The 3-year warranty card (US) is an extra $20, which seems fair. Just be sure you ask for it if that's what you want, because they may forget to offer it.