I shot with a Sony a7s for a short film yesterday, and I absolutely loved it. Forget Canon!!!!
But for real, I am thoroughly impressed with the a7s - low light capabilities, as many have pointed out, are incredible. The video is also sharper, and you can externally record 4K, along with internal 1080/60p. I'll probably keep a few Canon lenses around, but my 5D mkII is up for sale as of today.