I had to stay with an APS-C sensor because I have several EF-S lenses that won't work on a full frame camera, or I probably would have gone for the 6D (they cost about the same).
I got to play around with it a bit over the past few days. Color on the photos is just phenomenal. I miss easy access to scene modes, and manual mode take a bit of getting used to as the settings on my Rebel were in my muscle memory. The thing is FAST. Focus is near instant and silent. Shutter has a nice deep click to it. First shot I took, I thought the camera left the shutter open as I'm used to the two part click in the Rebel. This is just one smooth motion. Live view and remote control by iPhone and computer is all wireless and very impressive.
Can't wait to try out that high ISO on some night shots.