Re: 16-35mm f/2.8 L - Image quality issues
Photodo gives the 20/2.8 a 3.4 rating. Given sample variation, I'm sure that the typical copy could be a bit higher or a little lower. Nevertheless, 3.4 is not a terrible lens. It's the same rating that the very good 300/4 achieves in their tests.
I won't go out on a limb and say that the 20/2.8 is a top-notch lens either, but its image quality is competitive with the zoom from f/4 up. Not so at f/2.8 where it stumbles a bit, at least in my experience. It has reasonably low distortion (as a prime should), controls flare quite well, and is worth consideration for landscape photography, at least on the 1D Mk II. Stopped down, It's pretty nice. Not sure on full-frame since I haven't tried it there.