I have had a 7D for a couple years and bought a 6D a month ago as an addition. I love the fast frame and reach of the 7D for birds/wildlife. What's blown me away is the high ISO performance of the 6D. I never realized how much flexibility it adds. Even 3 stops higher than the 7D, the 6D's noise is much more attractive. I'm headed to Yellowstone next month, and look forward to getting the best from both of my cameras.
I use a 50D for birds/wildlife, but my experience with the 6D has been the same. I'm blown away by the IQ and high ISO performance and it is opening up new avenues of photography. I seriously lusted after a 5D mark III for the AF system and ergonomics, but I just couldn't justify $1400 more than the 6D, particularly without the GPS/Wifi (which I do use). I think a 2-format kit is a very practical compromise if you have the lenses to support it.
I'm also pleasantly surprised how much I like the build and handling of the 6D, considering I have large hands and I've used the joystick for 10 years on a 20D and then 50D. Canon did a solid job designing this camera for the intended purpose (travel, landscape and portrait shooting).