Considered an 'entry level' full frame, the 6D has photo quality equal (so I've heard) to the Canon 5D Mark III, but with less frames per second, less focus points, less weight, and about $1800 less dollars out of my bank account for the kit with lens. The sacrifices were easy to make to save all that cash, and I am beyond thrilled with the photos I'm able to get with this camera and my favorite 28mm lens!
ISO 100, 1/40, f/5, 28mm
I took this one and was impressed at the clarity in his face, so I zoomed in...
...and peed myself. I could see all the little drops of water in his hair, which just blew my mind. The only editing I did was the white balance. Most of the photos I take with this camera are super sharp.
The best thing about the full frame is the amount of light that hits the sensor. It's a crazy difference looking through the viewfinder of this camera compared to my crop-sensor Rebel XSi. It also has amazing ISO capability, up to 25,600 where my Rebel's limit was 1600! In the one above, I set the ISO higher at 1250 because we were shaded by a tree, and there is no noticeable noise in the photo. Win!
ISO 4000, 1/3200, f/3.5, 28mm
ISO 100, 1/250, f/2.2, 28mm
ISO 400, 1/250, f/9, 28mm
ISO 100, 1/160, f/3.2, 28mm
ISO 100, 1/80, f/2.5, 28mm
ISO 100, 1/250, f/2.5, 28mm
ISO 250, 1/160, f/5, 28mm