Just to add to what already been said, I think it's a great lens. Been using it for 18 months now mainly aircraft, but also wildlife. Here are a few comments that I've noticed:
* It's softest at 600mm (as you'd expect) but still very good and usable. If the lights a bit poor, hazy, dull etc then I avoid 600mm and zoom out a little and crop afterwards
* All the other focal lengths are excellent and the performance at 600mm is much better than other zooms I've used at the longest end.
* I mainly avoid shooting wide open and try f8 or f9. This isn't really a problem with the high ISO performance on modern DSLR's
* Unless I know a subject is really close I leave the focus limiter on, this really helps with the auto focus speed
* With my Nikon D7200 and a battery grip it's pretty heavy so I use a Black Rapid shoulder strap on to the tripod mount. That works well and I can carry it all day
* At 500-600mm technique for hand holding is really important regardless of shutter speed (to an extent)
* If your shooting aircraft on landing approaches and start at 600mm it's more than one turn of the zoom ring to get back to 150mm. It possible to do while following something towards you but needs practice. I try and rest my palm on the tripod mount and rotate the zoom ring with my finger tips!
Hope that helps.