I bought one directly from him to use with my Phase One back. The 680 is a great concept of a camera, and he put it on a monorail- in theory this could be the perfect little camera to allow full movements. Coming from large format, I really miss being able to tilt the camera back for effects, like exaggerating the foreground. There is still no camera on the market that allows for this- all Alpa and Arca offerings have front tilt only.
There are 3 issues that arose; first, the Phase One back did not seat properly, but after spending a few hours to raise the attachment with shims, got it to work. The second issue I did not think of was that the sliding back simply moves over to expose the Phase One sensor to open air. I should have had him attach an existing Linhof sliding back, but his back is much simpler; smaller and lighter. I ended up using my old Blackjacket dark hood with draw strings, which covers and protects everything. I can still make this work, I believe.
The final issue is that the sensor plane has to be exactly equal to the ground glass plane to focus, but it was not very close. He may have had it set properly for his similar Mamiya back, but it has to be exact, and there has to be a way to adjust it. The Alpa system has a set of very fine shims you can add to the back to make it perfectly registered.
Since he is in Korea, I could not just send the 680 and my back over to him for fine tuning. The solution I am working on is ordering a set of thin shims from S.K. Grimes that I will use to finely tune the sensor plane to match the ground glass plane. It is a work in progress, but many tweaks necessary to make it work properly.
If the 680 you are looking at is for attaching a 35mm camera, then these issues should not be any problem, they only occur if using a digital back. If you are simply looking to add a camera body, like an A7r to the 680, I would say go for it.
Hope this helps.