Summary: I bought this lens as an upgrade to my 16-45mm f/4 lens. The older 16-45mm lens has excellent sharpness, contrast and color reproduction, but lacks in build quality and under certain conditions shows significant color fringing. To my delight the 17-70mm equals or betters its older cousin in all optical categories, even with its extended zoom range. Also, the build quality is excellent and the color fringing is much better controlled.
Pros: Consistent Output, Durable, Easily Interchangeable, Sharp Focus, Strong Construction