As adapters already exist for the current E-mount line to enable the use of Canon (and Nikon) lenses, the A7 won't be a "game changer" at all. Especially if one starts using lenses that can throw an image circle large enough for a 24x36 sensor. The m43 format is great as the sensor is 1/4 the size of a FF chip, so lenses are that much smaller. But start wanting a 100mm macro or 15mm FE (our standard UW lenses), then you're no better off that using a DSLR with crippled (as per current technology) slower AF. When you start considering topside lenses like a 70-200 or more...then the camera becomes a liability than anything as it's too small a "handle" to properly stabilize the bigger lenses against one's body/face. Remember too, theoretically, FF lenses require larger elements, leading to more expensive glass.
I love my NEX-6 and my ability to use my Canon EF lenses with the Tech-Art adapter AF is still hit/miss with the on sensor phase detect. Navigating through menus can be a bit of a pain in the rear end and while soft keys are easily customized, it's not in the same league as a DSLR. Yes, it's easier on the shoulder to carry around, but when it's the "must get it shot" moment, I still reach for my 5dmk3 or 7D.