Not a downgrader, but, agt the beginning of the summer I was in search of a backup/secondary body - something to compliment my 5d3. After lots of reading about the 6d, I rented one in April just to see if it could keep up with the mk3. As primarily a wedding shooter, I needed something that would produce good images in low light, I knew that technically the sensor in the 6d could kick butt, but, I was worried about the AF, particularly how the outer points would work.
I have to say I have been pleasantly surprised by how well the 6d performs in the field. I use it without hesitation. And so far, my clients seem to like what the 6d can do as well. It';s about a 60/40 split in image picks (sometimes 70/30, depends on what lenses I mount and what the circumstances are, IE larger weddings both bodies get more use, smaller ones I favor the mk3 cause there is no need for using both bodies).
Even in the low light of the reception, with a flash mounted the outer points lock on! (take the flash off and its center point only, so I end up having the 6d be my on cam flash body, and the mk3 handles the off camera flash work because the AF i much better across all the points).
Yes, I do find situations where the 6d doesn't cut it without some assistance (external constant lighting, flash on camera). But I'd say for close to 85% of what I want to do on a wedding shoot, the 6d handles it quite admirably!
With that said, I wouldn't downgrade. My mk3 is awesome and does serve me well. But, if the 6d had been released last spring, and I was facing this decision with only a 7d in my bag and a limited budget - I would have no issues making the 6d my main body (until I could snag a mk3).
Different strokes for different folks though. If you need fps and shoot fast moving subjects, the 6d just isn't the right tool. For my needs though, I have been very impressed with what the 6d can do.