What was your reason for switching?
Since I purchased my Fuji x100s, I have been using it about 90% of the time I need a camera (mostly because I always have it with me). When I first got my hands on the Fuji, I went for two whole weeks without touching my 5D3. So it was a lot of money just sitting on the shelf. Problem is, I still have a lot invested in EF glass that I want to continue using when I need them.
I don't shoot sports or super dynamic action scenes (very often) and figured the body switch would put a little over a grand back in my pocket. After assessing all the different things I shoot, IQ, and what I actually need the Canon system for, I realized that I just couldn't justify having the 5D3 since the 6D offered me enough to get the job done at a reduced cost.
Things I miss the most are CF slot and AF system from the 5D3. I recognize that moving up from one of the crop bodies (minus the 7D) or 5D2, the AF of the 6D will seem perfectly acceptable if not good. However, moving backwards from the 5D3 AF system, it leaves much to be desired. That being said, I am still able to get the job done with what the 6D has to offer.
Also, I have been selling things here and there and trying to build up a fund to purchase something else. My hope is that Canon puts outs a body aimed at absolute IQ like some are speculating they are working on which is what my piggy bank will be broken for. My interest is also piqued by the rumors of an interchangeable lens full frame Fuji in the works. The possibility of both of those things has led me to want to recoup as much cheese as reasonably possible into the camera fund in preparation.
At the core though, the simple explanation is that I was able to take a long hard look at what my needs are, realized I don't absolutely need the 5D3 and took the opportunity to put some money back in my pocket as the 6D is capable enough for my needs (and most importantly provides the same IQ).