Re: Vignetting with the 5D
I don't think this is so much of an L glass issue, but the nature of wide angle to moderate telephoto zooms with a zoom range greater than 3:1. I've never had a 3:1 zoom lens of this type that didn't exhibit some vignetting, even on film. If vignetting and geometric distortion are of prime importance, I think it is best to stick to the 2:1 zooms.
That said, I don't think the vignetting with either my 28-135mm IS or my 24-105mm IS L is even close to "serious", I've only had to correct for it a couple of times since I got my 5D. Of course, as they say, "your mileage may vary"!