I just received my copy of the Nikon version, seems fairly similar to the Nikon 24-70 but a little wider with more barrel distortion at 28 and slightly more vignetting. Very sharp with and without VC. It definitely helps video stability. Stills at 1/20 are way sharper with VC. It is parfocal, the Nikon is NOT, which is helpful for video. This is my first non-Nikon lens, but it seems well built with good optics.
My question/issue is that I noticed that when zoomed to 70 with the barrel extended, if you look at the end that attaches to the camera, a lot of circuitry is exposed around the optics. This looks like the stuff for the VC with circuit boards, ribbon cables, stuff you would see inside a computer. Could any dust, debris, etc. fall out of this area and into the sensor of my new D4 during heavy use? It looks clean, but there is stuff further back along the barrel that could potentially enter the camera. For this reason alone, I am considering returning it. All my Nikon lenses are sealed where they attach to the camera.