I have owned both the EF 17-40 (two copies) and the EF 16-35/2.8II. The 16-35/4IS is in another league, especially in the 16/17-20mm range.
My copy has almost no chromatic aberrations, and no trace of jaggies and such behavior. As sharp as the EF 24-70/2.8II, but much better regarding field curvature, which the 24-70 suffers a lot from.
Wery reistant against flare, the best lens I have owned in that regard.
This is a lens in the upper league, also considering being an L-lens. Recommended!