I have a 5D Mark11 with some nice L lenses plus assorted others.
One thing that is important to me and many others is weight. I use the 5D for IQ period. Low light performance is also important. It has not failed me. But, at 66 I find the weight of the system (I'm a landscape guy and always carry a tripod) bearing down on my hikes and shooting. It now looks like I have some lighter options!
Right now, the X-E1 is my first choice. Much lighter and all reports indicate a IQ that equals or betters the 5D. Some issues remain re raw processing...
Yes, the GH3, NEX and Olympus systems are also very good, but the X-E1, with its manual aperture and shutter controls calls to me. Yes, the Pro model has an optical viewfinder but I find this just a complication (eventually to be abandoned JMHO) and not needed considering the quality of the EVF.