Acceptable Zoom Lens
I purchased this first generation of Canon EF 24-70 / 2.8 L USM along with Canon EOS 5D (first generation) in 2005. I have always been a Nikon user until then, and took the opportunity of trying Canon dSLR system for the first time - although, I own several Canon PS cameras as well. I did not own any dSLRs of Nikon because none had FF sensor in an affordable range, and I decided to take the plunge for Canon just for FF sensor alone.
The lens performs acceptably well on 5D. I have had no big complaints. However, the softness becomes somewhat apparent in landscape shots with aperture setting of <4.0. It does not become that much of an issue, to me, because I almost always use f/5.6 or smaller.
I often shoot rectangular objects like trains and buildings. While the zooming range is generous, I have to watch out not to use lower end of zooming range when shooting trains. 28mm is fine, but 24mm tends to emphasize perspective and barrel distortions, easily deforming the original shape. However, this isn't really a unique problem for this lens - all wide angle lenses have this trait, more or less.
For the premium I spent on this lens, the constant aperture is always a welcome feature.