I have an MF version of the lens (assuming the AF version doesn't use floating elements or something).

Mine is quite good. After the Novoflex T and Leica Telyt 400mm lenses (despite their field curvature), it's the best 400mm I've used.

Probably more modern and much more expensive lenses can do better, but for most non-professional or non-specialised users of a 400mm, it should be quite enough.