I don´t do much bird photography and didn´t have the time to plan other tings (such as swiming competitions that I have some experience with), but here are some first impressions used on K-3:
Sharpness: Good, even wide open, but of course even better stopped down.
Flare/high contrast: Very good, no CA or other errors as far as I can see, and the contrast in some of these pictures were high.
Focus: exact, but I didn´t really have the chance to test the speed.
Vignetting: None (as expected if it is prepared for larger sensors).
Mechanically: Solid, not as heavy as I expected. Possible to use without a tripod, at least with shake reduction.
