DXOMark has completed their review of the Sigma 500mm f/4 DG OS USM S supertelephoto lens. Like most of the new Sigma lenses over the last three years, this one performs quite well when compared to the Canon version.
From DXOMark
Head-to-head against the Canon EF 500 f/4L IS II USM, the Sigma 500mm f/4’s image quality compares very favorably. The Canon lens is fractionally sharper on the 5DS R, but the Sigma offers a modest improvement in resolution on the 1D X Mark II and 5D Mark IV. To be fair, there’s very little to separate them in real-world terms on any of these DSLRs, however, making the Sigma 500mm f/4 a great value option. Read the full review
We're not sure how many of these Sigma is going to sell, as it will be quite difficult to sell a $6000 lens for a Canon system that isn't a Canon product. If you own one, sound off in the forums about it's real world performance.