Visitors during the weekend had a 6D with them. They allowed me to test it with the YN-622Cs.
I love the quick panel style external flash controls. Much faster to use. I used 580EXII, 430EXII, 550EX and YN568EX to test functions.
Flash mode, Shutter sync, FEB, FEC, FEL, E-TTL mode, Zoom, Wireless modes (Optical/Off), Master flash disable, channel, firing groups, Ratios and AF-assist beam all worked as expected.
The only gap was the modelling light. I couldn't find a depth-of-field button, and there were no results from a Guide search for "modelling". A later study of the user guide showed that the depth-of-field button was now on the other side of the lens mount! There is a reference to "modeling" (sic) which implied that the facility was in fact available, but I was out of time to test. The visitors finally wrenched the 6D from my grasp.
It is a full Class 1 Flash pass, with praise for the much improved menu interface. ("The Other...Guide" will be updated to show the quick flash control panel.)