I changed my Nikon 200-400 for a brand new released Sigma 150-600 for Nikon. The lens is just new on the market for a very acceptable price (compared to Nikon prices). I'm surprised about the Auto Focus, it fast and spot on even with fast moving objects like smaller birds. Sharp images are always very important to me so I'm very happy with the results so far and can really advice others who are looking for a light and good quality lens.
Here are some recent images taken with the Sigma 150-600mm-f5-63-dg-os-hsm-s http://www.sigmaphoto.com/product/150-600mm-f5-63-dg-os-hsm-s