Well my new 6D just gave me a couple of scares... I've had it for a couple of days, but had a lot going on in life, so barely got to use it just to test out if it even works.
Well, tried the low light shots, didn't focus in darkness, but could still focus way lower than my t2i can. Those shots though, they seemed to be kind of really mushy, like almost smudged out. At that high of ISO, 5000-8000, (my t2i never goes above 3200), i figured, maybe i'm seeing something weird on the LCD screen, and on the computer they will not look so mushy.
So when I got home:
1. Loaded up the picures, and they ARE mushy/smeary, and I'm like what the heck, whats wrong, is it the 6D, or did I get a REALLY crappy copy of the 24-105 (from testing it on some rulers/newspapers and whatnot, the focus seems to be pretty good on the center point). After playing around with options and stuff, turned out, I had the "High ISO Noise Reduction" option set to Medium.
Turned it off, and the images started coming out much sharper and clearer. A touch more noise, but its SO much prettier than my t2i's noise. On the t2i even at ISO 1600 or so, the noise starts being very obvious and very colored, but on the 6D, 1600 without any reduction is still quite clean, and even higher up the noise is just white grain, much more pleasant to look at than the red/blue/green noise of the t2i.
2. Second was, I started noticing a white dot, almost a star, right in the middle area of all the pictures.... well that got me worried. After a bit of research, found you can do the manual sensor cleaning with flipping the camera face down, and it seems to have mapped that dot out of the sensor.
Feeling much better about my purchase now
And sorry about the ramble.