Having a MkII since it came out, very happy with it. I have the 5D 1 as well, but would prefer a 6D as a back-up/second camera. Unfortunetaly it isn't weather sealed. Totally incomprehensible for a camera directed towards landscape and travel.
5D III wouldn't add much quality, and I don't need the speed/focus. Slight chance I might do something crazy and buy a 1dx, but I think I'm going to wait out what the future might bring. Canon needs a big step forward again, they are running behind on Nikon (less functionality and specs for a higher price tag).