Conclusion
If you are in the market for a new wide-angle lens, I definitely recommend spending the extra $250 and getting the new 16-35L. In terms of sharpness, contrast, and extra features, you will get your monies worth. A very welcome improvement is the ability to focus at 0.28m/0.9ft. In addition, for those whom already own a 17-35L and are disappointed by its performance wide open, now you have a viable choice to achieve sharp images at that aperture setting.
Overall, this is one of the sharpest super wide-angle zoom lenses that I have ever tested. I recommend it!