Well I decided to rent the Canon 6D and shot with it at a car show today using my old 28-80 L lenses. Wow, did the pictures stink! Vignetting on almost all the pictures and none of the images were really "sharp". I have the camera until Monday and will continue to test it with my other lenses on Sunday.
I was really hoping I wouldn't have to replace my 28-80 L with a newer L lens. It's great on a APS-C body but is horrible on a FF camera.
Did you run AFMA on this lens/body combination? If not that might help. There are a number of good on-line instructions on how to perform this.
What is AFMA?