My first 35cron has a few very light cleaning marks that I wouldn't have noticed if I hadn't done the Rockwell lens test. My second one is totally spotless and the only issues were getting the focus tab replaced and getting the aperture ring back into place (Once I get it back from lacklands).
I thought that my 16-35 had a small gauge on the front element from when the UV filter smashed in, and instead of trying to remove the glass first I went right for trying to remove the filter for the longest time and there was no lack of sharpness. Of course, when I examined it before selling it, I actually couldn't find it. Maybe Canon glass heals or maybe it was a hair, I don't know, but I gave the seller the option to return it within a reasonable amount of time if they weren't happy.
My 55-200 from canon has been absolutely abused - dropped, filter smashed and fine glass all over the front element, some removed, other bits probably rubbed into it, knocked, the barrel is cracked, and the lens is only slightly softer than when I got it. Maybe on a better lens the difference would be noticeable, but...
The kit lens that came with my canon has element separation or something by the front element, and I haven't seen much difference (Actually, when compared to the 16-35, the L lens was only slightly sharper).
I seem to keep my primes in better shape than my zooms, don't I?