Maxotics, yes, I actually ordered a cheapo "dumb" Nikon adapter for my AIS manual focus lenses before I rented the A7S and they worked fine. The adapter was a bit of a kludge, though -- making the lens longer, of course.
I'm looking forward to playing around with thrift store lenses. I have a few Meyer Optic screw mount lenses from my late grandfather's East German Practika that I can't wait to try out.
The thing that is really selling me on the A7S is the ability to get manual focus aids when you want them -- even while recording.
If you're a sports or action photographer, DSLRs are the way to go. But if you don't want attention as "the camera guy" with a big phallus of a camera in someone's face, then I really believe mirrorless systems are the future of photography. Full Frame DSLR primes and lenses keep getting bigger and bigger. I can also use Leica or Voigtlander lenses to keep things small.
The A7s, once I got used to its quirks, was a joy to use for a weekend. It made me want to use it, to walk around the city with it. Unlike a DSLR with a big honking 24-70 2.8 lens. I actually liked the Zeiss 24-70 F4.
I would like the bigger grip on the A7II, but I can live with the A7s as is. Based on Andrew's report, the IBIS doesn't seem to be the killer app, and may even add unneeded complexity and moving parts.
For me -- as a street shooter, sometime film maker, and general hobbyist, the A7s is the perfect balance of versatility, quality, full frame, and actual LIGHTNESS that makes me want to take it around.
I hear D700s have become cult items, so it should be easy to sell. I will keep most of my AIS primes, though and sell the big heavy glass.