What I like about it
Over the 5DIII - It is cheaper (and I am realistic enough that this means certain features are given up)
Over the 5DII - It is newer. It is smaller. It is lighter. Has GPS and WiFi. And I know it is still not proven but I am sure better IQ. Takes my SD cards (I do not have CF cards)
Over the D600 - Well it takes my EF lenses and EX flash , so really the rest of the spec list is a moot point
Over the 7D - it is FF
Over my 500D - it is FF. better sealed. better IQ. better focus. better view finder.
Seems we're on the same boat. I'm not dissatisfied with my 500D. I can get the best pictures out of it even if it's outdated. I just want to go FF and its better IQ. 6D is it. I'll just wait a bit more for prices to simmer down. If I have Nikon glass and flash, my decision would be different.