My 6D arrived few days ago.. and I can say.. WOW!
comparing my 6D with my old 40D and 1DS mk3:
6D focuses with more accuracy than my 1Dsmk3 body and 40D, I did many test shots.. using 2 lenses: 35mm f1.4L and 135mm f2L at day and night, no external flash, just using one shot mode and center point focus, only!
here we go..
1DS mk3 90%
6D 94% + accurate focus, dead on!
40D 2% + focus hunting
1Ds mk3 12% + focus hunting
Chuck Westfall from Canon USA says:
The 6D is 95% improved over the 5D II, specially in the center point AF down to -3EV, and, it refreshingly gets the shortest Canon DSLR name we've seen in many years - one confirming a new model line. He emphasized that Canon's goal for the EOS 6D was cleaner pixel-level image quality. "From what I'm seeing, they were quite successful. The 6D advantage builds as the ISO setting increases. The difference becomes rather easy to see at ISO 3200 and the 6D's lead continues building until it leaves the 5D II in the dust at ISO 51200."
Bryan from the-digital-picture says:
I can focus the 6D's center point on a subject with reasonable contrast down until auto exposure gives me a setting of 160 at 10 seconds and f/2.8 (really dark) with the Canon EF 24-70mm f/2.8 L II USM Lens mounted (note that the 6D's metering range spec is listed at EV 1-20). The Canon EOS 1D X's center point could not focus on the same subject with the same lens mounted. Note that the 6D focuses very slowly under these dismal lighting conditions - but locking slowly is far better than failing to lock.
My conclusion is.. the Canon 6D is one of the best cameras ever made by Canon.. why?
1- phenomenal center point AF system, down to -3EV, just awesome!
2- very low noise in high ISO, best from any Canon camera to date
3- full frame in a lightweight body
4- low power consumption from a SDHC (and affordable) card
5- silent shooting mode
6- white balance is outstanding.. the color tone BLOWS AWAY from my 1Ds mk3 and 40D
7- first camera with wi-fi and gps built in..
8- made with plastic?.. oh yeah! (but don't forget, just the top of the camera uses plastic, for the gps and wi-fi reception signal).. The Canon 6D uses a ABS (Acrylonitrile butadiene styrene) and polycarbonate mixed.. "Polycarbonate / ABS is a durable material. Although it has high impact-resistance, it has low scratch-resistance and so a hard coating" by Wikipedia