Cinema5D has an informative first look:
In comparison to the Sony A7S in 4K it’s really really hard to tell the difference and I dare you to try. The most apparent difference is the A7S running at a base ISO of 3200 and the Sony A7RII running at a base ISO of 800. You can see the grain and compression / noise reduction artefacts on the A7S on moving images. The Sony A7RII looks a bit cleaner. As a still they look virtually identical.
EOSHD has it’s typically enthusiastic hands-on report:
The 5 axis stabilised 42MP sensor appears at first glance to be a game changer.
I have never known such a well balanced, high performance chip.
It looks significantly less noisy at first glance than the Canon 5DS. I say Canon have a lot of catching up to do.
Jon Roberts has posted an ISO comparison chart (against the 5D Mark III and the A7r) on flickr.