On Copy number 5 I believe, 3 new and 2 used, I tried 2 used ones first, one was decentered and the second had focus issues. Then I tried a new one that wouldn't focus properly (and I mean way out not just out of tolerance like in the lens rentals article) so I exchanged it for another that again would never be sharp on the right edge of the frame at any aperture and I sent that back for a refund. I then bought another new one from a different retailer which is soft on the left side and totally misses focus sometimes and every now and then get's it spot on, not really sure what to do at this point, the lens is supposed to be amazing and I want to use it I just can't seem to get a good one! I had considered moving to a 15-85mm but apparently they are just as bad!
I have also had problems with sigma 18-50mm f/2.8, always fantastic optically but never focus well.
My 70-300mm L MIGHT have a TINY centering issue but it seems to come and go with 90% of shots being tack sharp corner to corner so might just be related to the IS system in some way, in any case I don't think I would send it in either to replace or repair because the results are so stunning already I don't want to risk losing what I already have!