I got one of the JAS adapters and have been testing for a few days with C645 35 & 55mm lenses on D800/810.
Adapter is well-made, fits well. As others have said, AF is accurate - if noisy and slow compared to native lenses.
One downside, there is no AF in live view. Not why I bought it, but I didn't realize this beforehand. The maker says he is not sure how to implement this. Also, the focus will sometimes move when turning on/off the camera. I haven't figured out the cases where this happens, but something to be careful of when shooting for compositing.
On the Contax lenses, I've been mixing them in w/ Nikon nano-coat lenses (PC-E & 14-24). The Contaxes are sharper in the corners wide open (un-shifted 135 format), and comparable stopped down. Way better than the older-generation PC-Nikkors. Color is warmer, more magenta than the Nano-coat Nikkors. The Contaxes seem more susceptible to veiling flare from broad sources (open sky above, open window to the side. It's often subtle, and can sneak in if I'm not paying attention, just giving the impression of lower contrast. Generally though, contrast & color is excellent compared to the Nano-coat lenses, maybe slightly lower contrast but I'm still evaluating this.
At the moment I'm still waiting on some pieces to allow shifting the Contax lenses, so have no impression beyond the 135 frame yet.