I must say that is probably the best deer shot I have seen in this forum. The subject isolation from the surrounding trees, but still very much part of it, is excellent. Seeing the photo first I thought it was taken with your 400f2.8, but was surprised to see it was taken with the Sigma. Obviously a good lens, in the right hands of course.
This is one of my best deer shots to date. The Sigma is a very good lens I just dont get it on the camera very much. It is much easier to use from the drivers seat of the car. I think this lens (especially the new OS version) is one of the great values in super telephoto land. With the 1.4TC it is quit comparable to the 200-400VR at half the price.