It looks stunning but there is not much of a difference in terms of quality, it has more to do with the lenses that you are using and the settings (the blurry background is possibly the biggest difference if you are using a 50mm lens)
Beautiful image isn't necessary for camming and it is quite a hassle even if you have a powerful processor to handle it. My main problem is comfort and risk.
The c-920 is tiny, I can move it around easily, pan, change the angle, etc with my hand. My 70d is heavy, requires a tripod, won't rest on top of my computer so it needs to go next to it creating a weird "voyeur" sort of angle and making it so I have to switch my gaze from the camera to the screen constantly if I want to keep up with chat, and I just can't move it around easily.
Plus I am clumsy and knocking over my $1300+ camera+lens combo is just not something I am willing to risk. Why go through all that hassle when the c-920 is more than adequate for the task?
Edit: also, if you are on a mac which is what I like to use for camming you would need to figure out a way to do it, possibly with a special video capturing device. I don't care for camming with my PC because there is no iGlasses for Windows that I know of and I dislike the alternatives.
