I have a couple of GX680 II bodies and the 65, 125, 180 and 300mm lenses. I used this for many years with my P25 and a Kapture Group One Shot solution for wake-up and sync. This system rocked and was my go-to platform for my food and still life work.
When my batteries for the camera died and I discovered I could not buy new ones, I simply cut the base off one of the batteries, took the top off a Nikon charger, glued the two together and Viola! I now have a battery adapter that takes Nikon batteries (pinched from the D700 gathering dust in my cupboard).
Once I replaced my P25 with a P65+, and now the IQ180, I no longer need the Kapture Group One Shot solution. Using the backs in zero latency mode works fine. As I have also moved from V mount to M645 mount, I bought one of the Chinese adapters available on eBay and this works fine.
The 300 suffers from vibration issues due to the massive mirror, even when mounted on my massive Foba head and large camera stand. The other lenses are superb and have large image circles too. This is one of the most versatile systems around, but you have to get your head around the sheer bulk. I bought my system new in 1995 and it served in a high-volume studio environment for 7 years without missing a beat. For sheer ease and speed of use when using shifts, it's hard to beat. Full frontal movements with reflex viewing - what's not to love about that?