On fred miranda forums, Alex Phan posted a few image samples:
Using Pre-Production lens. Not final release.
www.fredmiranda.com
Granted, these are only small size images, but they give a good impression of what to expect.
I had the Sigma 500mm f4S in Nikon mount for a year, along with the Nikon 500PF, and that is an older lens, but Sigma does know how to make a very good prime lens.
Reports are that auto focus is very fast and accurate in c-af. The body uses a modern composite material, hence the light weight.
I'll pre-order the lens, as I don't care for a TC on a f5.6 lens, cropping does the job on the high resolution A1, and I have no urgent need for faster fps than 15.
It had to be Sigma that brings a Nikonesque lens to Sony e-mount, although granted, the Sony 300GM is a truly innovative lens as well. But for birding and wildlife, I feel the Sigma is the better choice.
I can soon enjoy the "ridiculous low weight and small size" telephoto shooting again that the the 500PF offered on the D500.