I apologize for not reading all the responses so far.... I'm sitting in an airport in Panama! LOL!
But - I just spend 2 weeks using the 600 f6.3mm photographing small birds in Panama. Obviously I have not gone through all my images, but my impression is that the lens is excellent considering the size, weight and cost of the lens. I often shot in crop mode and again my impression is the image quality is very acceptable.
That said, the f6.3 is a slow lens that forced me to shoot at high ISO's in the rain forests during heavy rains. I believe I will still have usable images once I judiciously apply noise reduction. Also, with some backgrounds containing high contrasts, the bokeh was rendered more contrasty and a much faster f4 lens.
In fairness, I usually use the 600mm TC f4 lens, and really missed the faster lens. BUT - that lens costs of several thousand dollars more is an additional 4 lbs!
In summary, I think the lens is a great alternative to the big, expensive 600mm.
Hope this helps.
Karen
