The A7R is very intriguing. Before I moved to Canon DSLRs I was on Micro 4/3 and this looks a lot like shoehorning a double-size sensor with similar pixel density into a similar body. The problems are the same, though: contrast detect autofocus, while potentially extremely accurate and quite fast for static subjects, is abysmal with moving subjects. This is the whole reason for development of on-sensor phase detection, which the A7R does not have.
When Canon rolls out a new hi-res sensor in a traditional (big) DSLR frame with excellent AI Servo, those who jumped on the A7R might be sad. Even though the A7R body is small, its full-frame lenses will be big, cancelling its size advantage. For now, I'm more than happy with a 5D3.