You just need some luck to receive a good copy.
The tolerance of the Canon lenses is very big. Especially with the wide angles.
There is also considerable field curvature on the 16- 35. I am using a good original mk1 copy , but it was the second one I tried. When I tested them I realised they didn't focus all in the same place across the frame, particularly at the widest settings, and the corners could be in focus at a much closer point than the centre.
I also tried the 17 - 40 but the copy I tried seemed worse than the 16-35 and not what I expected at all as that lens had some great write ups and I had seen some great shots done with a friends lens.
I am still using the Mk 1 lens and it seems to work better on my 1ds3 than the mk2, but has never been as bad as some suggested although that could well be sample variation. I haven't changed as it means using a different sized polarising filter than all my other lenses and the hassle doesn't justify the small increase in quality I would get with the MK2 lens. Many of the issues only happen at the very widest settings anyway and I think my money would be better spent on a 14mm prime.