I just took that lens through it's paces at DLR for close on 2 days. I deliberately set out to keep this lens on my camera for the two days; rather than reverting to my normal walkaround lens.
My take on the lens after these two days is as follows:
I don't like the way it hunts for focus and the lens was constantly trying to focus and re-focus.
I think it's a great lens for taking pictures of static displays or when you have time to work through the focus point i.e., if you're standing still and the object is stationary.
I found the focus/refocus to be a source of frustration for me when I was on dark rides. And yes, I do shoot with single focus point. I also normally have my canon camera set to AI Focus. I did move it to AI Servo to see if that made a difference in the dark rides. This function made a little bit of a difference and the lens was a little quicker at getting into focus but I still found the lens had a tendency to focus/refocus.
In the end, I moved to manual focus rather than automatic to try to stop the focus/refocus. As a consequence, most of my pictures are blurry as I am myopic and have astigmatism (it definitely has nothing to do with the fact that I am @%$%# at manual focussing a lens

). Still, I did come home with enough pictures to sufficiently record the trip.
I acknowledge that I'm just an enthusiast hobby photographer. I expect better photographers than me will have much better success with the lens.
I also have a Canon 17 - 55mm, f/2.8 in my bag and I know what that lens is capable of. I don't think the Sigma measures up to that lens at all. It took all my willpower to keep the Canon in the bag and persist with the goal of giving the Sigma a real go for the 2 days I was at DLR.