I got mine a few days ago. Carried it for four hours today at the zoo. Seems much lighter and smaller than my 400 2.8. To have the freedom with the focal length is amazing. I would have never gotten half the shots with the 400. Shot a swim meet earlier during rain and and overcast sky. I was worried about a 4.0 vs 2.8 lens. Not a big deal with the 1dx. Used 1000 ISO and could keep the shutter speeds above 1000. I think I may sell the 400 2.8 at this point.