I just came from a local camera store in Dallas with this lens; I'm using it on my 1D series. I have to say this thing is impressive. I've always dismissed sigma in the past for their build quality and lackluster performance from the few I've tried out.
Build quality is better than my L lenses (much like a zeiss lens really), the colors out of camera are excellent and focus is spot on for me.
My main concern going is was focusing so I played with it in the store and purchased it after micro adjusting it a bit. So far I've tried indoors under fluorescent lighting and in some very dark rooms, outdoors in bright sunlight and heavy clouds I've tried many different situations between f1.4 and 4.0 and focus is great. Before micro adjusting it I had a small bit of front focusing but now it hits it right every time.
I'm blown away by this lens, I've always been an L glass kinda guy but I could see being a sigma owner if they made more lenses like this.