This lens is amazing for the price. It is sharp in the center and across much of the frame wide open, but one stop down it becomes tack sharp. AF is excellent in both one-shot and servo tracking modes. Takes the 1.4X extender extremely well - negligible hit in IQ and AF performance.
Very fast and accurate AF; Tack sharp; Excellent contrast and colors; Smaller/lighter than the great whites and much more discreet; Very smooth background blur at wide apertures
None - especially at this price