I bought this lens used based upon on the glowing clamor it inspires on internet forums.
The major downsides one reads about the lens is the center softness between 250-300mm (wide open - it supposedly gets better one or two stops down), plus soft corners at these focal lengths that stay relatively soft compared to the center, regardless of aperture.
The first thing I did when I got this lens was mount it on a tripod and test these attributes using live view for critical focus. Testing at four different focal lengths (70, 125, 200, 250, 300), with test shots done wide open and f5.6, f8, f11, and f16, I discovered the consensus is mostly correct, but I also learned that determining this kind of thing is just as prone to focus error as it is to alleged softness.
When you use live view (D700 in my case) and zoom in to the pixel level to establish critical focus, the first thing you will notice is that the tiniest smidgen of focal adjustment can radically alter pixel-level sharpness. The second thing you will realize is that no autofocus system can establish focus to this level of precision. The third thing that will enter your mind is to question how much the anecdotal evidence about the sharpness of this lens (or any other lens for that matter) is due to tiny focus errors, which wider apertures exacerbate.
Having said that, I concluded that this lens is actually quite sharp at 300mm and f5.6 at the center – definitely acceptably sharp, if not just plain sharp. But it is not as sharp as it is at 250mm and shorter. This corroborates the collective opinion on this lens. And yes, the corners at 300mm lack somewhat wide open as well, but they do get remarkably sharp even up to f16.
I also know that handholding this lens and chasing things like birds will never garner the tripod-live-view level of focus precision, so there is some merit to avoiding 300mm and f5.6, but you know what? I just did that with some flying geese and I was blown away by 300mm and f5.6 – when the camera and lens nailed focus. F8 was a little better.
Highly recommended. Even at 300mm and f5.6.