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Not a lot I can say that hasn't been said, but I will confirm what others have said. When I first took it out and tried it in the yard, it didn't seem to focus any faster than my 200-500 - which was one of the reasons I'd purchased this. I've concluded that the light wasn't good enough on that overcast late afternoon. I took it down to the lake where I like to shoot, and it was amazing! Fast focus, and frame-filling 600mm. When I looked at the shots on the computer, I was even more pleased. I've been happy with the sharpness of the 200-500, and just applied a little sharping in post processing, but these shots were absolutely sharp right out of the camera! Superb lens! Today I tried it with the 1.4 extender. Still pretty fast focus, and seems quite sharp - at least as sharp as the 200-500, and maybe even a little more so - and at 840mm!!!
Another thing to add after 6 months of use: I have this attached to a D850, and _always_ shoot hand held. That's with a grip and extra battery on the D850. I'm a 150 pound 5'10, so you don't need to be muscle-bound to use this lens hand-held! I walk up to four miles with this thing (okay, walking is my exercise, so that part isn't unusual), but just carry it at my side with a Peak Design slide strap, and swing it up for shots! It's that easy to manage!
Possibly the sharpest lens ever made. The image stabilization is off the charts. I used it extensively photographing wildlife in Alaska. The weight reduction compared to the older versions is welcome. One of the best lines money can buy.