I recently put it through my first field test with a couple of guys from up north here in Greenland on a dogsled trip in the Disko Bay area, and I must say I am very impressed with the images at all focal lengths and shooting it wide open.
Attached here are two shots. Both are shot at ISO 640 and 1/2500sec wide open at 400mm (f/5.6) and 164mm (f/5.0) respectively, on a 5D III (in minus 23 Celsius

). Not a whole lot of sharpening in Lightroom was necessary.
(Edit: The second shot of the dogs was cropped to about 50% of the original frame).
Going through my shots from this trip I had so many keepers even from a very bumpy ride on the sled, doing lots of zooming in and out and having to respond quickly to changing speeds and conditions. I must say I am very happy with this lens - and it kept my 70-200 in the bag most days because of the flexibility of the zoom range (we had good light and I decided to make the DOF compromise to get the range instead).
So in short: I don't see any sharpness, IQ, IS or other issues with this lens. It just performs.
Full size jpegs are here: