I've had this lens a couple of days now. It's been purchased to photo a cricket match for a school where the parents have requested some shots of their kids in action. I will try to sell the images I take to the parents. Hopefully I will cover the cost of the lens. In my tests I have been really surprised how good this lens is considering the cost. Even at the long end and with an F stop of 6.3 this lens delivers. At this aperture the images are slightly softer than at F8 but are perfectly usable and with a bit of USM look great. Sure, it is not built like a Canon L long lens and has no weather proofing etc but on the other hand it doesn't weigh like a Canon L either. As others have said, the OS is noisy but works. I'll be interested to see it's effect on the MK III batteries. No doubt this lens needs good light to get the best from it, and I would imagine that it may struggle to achieve quick AF in gloomy conditions.
I have never owned a Sigma, and perhaps I have been lucky and have a good sample, but based on what I have seen so far I would say that this is a good lens for anyone starting out in wildlife or Sports photography who needs a long lens but cannot afford the price of Canon L glass.