So i´m in the market for a full frame camera right now. I only have FF lenses for EF mount, so switching would at least mean the loss of a lens or two (money reasons).
Right now canon is far behind others. Or lets just say it: Nikon.
The d800 is 2300€ the 5d3 is 2900€ and the Nikon just is the superior camera. If you downsize the image even high iso is awesome, af is good etc. It´s just a really good body and much more in the price range of a 6d.
I want to stick with canon but if they are going to make their first competitive body in like 5 years i really have to consider switching....
Canon's sensor tech is behind sony/nikon but its not as bad as some make it to be.
What really set's canon apart is the EF lens selection. Its simply the best, excluding the ultra-wide department. The d800 is a great camera but, overall the 5D3 is a better package.