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.
Bodies: 6D, EOS-M
Lenses: Rokinon 14mm 2.8, 50mm 1.4, 85mm 1.8, 100mm 2.8L IS Macro, 135mm 2.0L, 24-70mm 2.8L II, 70-200mm 2.8L IS II, EF-M 22 2.0, EF-M 18-55mm, Extender EF 2xIII; Speedlites: 600EX-RT(2x), 430EX II, 90EX