One more opinion.... I shoot pro dance, and have a 1DMkIV, 1DMkIII, 5DMkIII, 5DmkII & 7D ... My flashes are multiple 580EX & 580EXII, and recently three 600EX-RT's. I have multiple PW TT5's and know about HSS. My big gripe on the 5D's is the 1/200 x-sync
. I have read all of the above.... Most of the time I only use stage lighting is x-sync is not a factor. There are times that I have to shoot a large group of dancers under poor or very low/ uneven light (hence DOF and aperture setting are critical) and often they are quickly moving and require a 1/200 to 1/250 minimum shutter speed.
Frequently I have to use high iso and the 5DMkIII is amazing. I looked at the 6D as a backup for those situations.
On the rare occasion that flash is used, HHS can be problematic with insufficient power if any distance is involved. As I am usually shooting in a theater situation, using addition flash units are not usually possible. Small increases in shutter speed make a big difference. The difference in a spin if the hand is sharp or blurred. (sometimes the blur is ok)
I realize that the 6D's 1/180 shutter is probable due to physical limitations of a less than premium shutter unit (100,000 life, max 1/4000...) and will not affect most users. The reduction from 1/200 to 1/180 is small but if you compare 1/180 to 1/250, it makes a difference.
I would have expected 1/200 and a 150,000 life from a $2K FF camera.
If I decide to get one, would probably be used as a personal travel cam, which I think is one of the targeted uses. Wi-Fi without Eye-Fi is nice along with a faster SD reader.