The Sigma 120-300mm f/2.8 DG OS HSM S is impressively well built. It is one of the new top-line lenses like the impressive Sigma 35/1.4 DG HSM. It also offers an appealing 120m - 300mm zoom range, far more flexible than a fixed 300mm prime lens.
My first batch of review coverage on the Sigma 120-300mm f/2.8 DG OS HSM S is now published, including commentary on size and weight in the field, with more to come.
The Canon 200-400mm f/4L IS USM is over 3X the price, a full stop slower and probably optically a solid bump up in performance, but not everyone has $12K lying around, and 120-300mm is perhaps a more useful range for all around shooting, all depending of course.