I shot one wedding with the 5D3 + 5D2. A week later, I sold the 5D2 and got a second 5D3. 
Not a huge bump on IQ and certainly the 5D3 isn't "clean" at higher ISOs. I try to keep it between 3200-6400. 12800 is my absolute limit. But it's great to have those options.
I think Canon got the AF right on this one, regardless of there the 5D3 lies in the product line hierarchy.





