I am more than happy with the optical performance of the lens. Very sharp wide open at the center, and the edges (on the Mark IIn anyway) is vastly improved over the original version. As with most wide angle lenses, this one can focus really close. So, in a pinch, you can use it as a mini macro lens, which was what I did to photograph the butterflies in the Butterfly Conservatory in Niagara Falls. I meant to bring my 180L macro lens, but forgot to at the last minute. The 50mm f1.2L that I also brought has an even less magnification ratio than the 16-35.
It's normal to expect great colour and contrast from Canon L lenses, and this one is no exception. Distortion is well controlled and focusing is very accurate, at least with my copy. I am surprised to see that the bokeh is actually quite nice, for an ultra wide angle lens with three aspherical lens elements. The size is similar in both versions. The only thing I am complaining about is the huge filter size at 82mm, from 77mm of the original version. A good 82mm circular polarizing filter could put a small dent in your bank account.
Overall, it's a great lens, albeit expensive. A worthy upgrade from the original version.
Niagara Falls looking down from the 20th Floor -- Canon 1D IIn & EF 16-35mnm f2.8 II. Click picture to see larger.