^ I use it with a 6D. I actually got it as my main lens, but did use it a bit on my crop 50D before I sold the 50D. Due to the crop image circle, the 50 was definitely sharper on the 50D, but you'd have to pixel peep. One thing you lose with using it on crop is the intended wideness, and the extra dof (or is it smaller?) using the lens wide open on a FF camera.
If you notice issues with your 6D without pixel-peeping, then there's likely an issue with the lens.
Fuji X-E2 | 18-55 | 23 | Nikon 60mm AF-D Macro | 55-250 |YN-560EXIIx2 | YN-565EX | YN-622C | YN-622C-TX