Probably the best lens i have ever used. Sharp at f2.8, and even sharp at f4 with a 1.4 x TC, with the sigma 2x you only need to stop down 2/3 to f7.1 to get exceptable images. The bokeh is creamy smooth, the contrast is good and the colour rendition has none of the old sigma yellow cast.
Very Very sharp; great colour; fast AF; "cheap
Not weather sealed