I had a 6D. Excellent camera, great IQ, small size (for a FF camera), best high ISO performance of any camera I've used thus far.
Ultimately, however, I gave it up for a second 5D3 once the price on the latter came down enough. Why? Mainly AF performance. Although the 6D worked well for still subjects and landscapes, once I started shooting more events, its shortcomings became more apparent. The outer points just aren't that dependable in less than bright light. OTOH, I've found that the 5D3's AF rarely lets me down. My keeper rate is noticeably higher, and that to me more than makes up for the larger size and SLIGHTLY more limiting high ISO performance (still pretty good however).