I'll keep this fairly brief, but I had the chance to use a (beta) GH5 with some firmware that had a much more sensible version number today. (Apparently the firmware arrived yesterday.) Here's what I have to add to my previous comments:
(1) It has (finally) gained a menu option for AF control.

Speed is as you'd think. Sensitivity is how much it tries to hang on to the current subject, or not.
(2) The video tracking in Locked-on sensitivity is impressive, even if someone walks in front of the camera.
(3) The video AF at short range is probably worse than before (which I hated). Do note this was indoors and so not that well lit. Changing from 5m to 1m it took 4 seconds to achieve focus, passing through correct focus and overshooting twice before getting it at the third attempt. This was repeatable. Anything under 3-4m wasn't good.
(4) I like the way the IBIS works with adapted lenses with no electronic connection. When you turn on the camera with such a lens attached it pops up a dialogue asking for the focal length. It also stabilised my 350mm "legacy" lens really well (that's 700mm FoV and very hard to use on a GH4).
(5) The menus have changed a lot, complete with dialogues explaining their function as you scroll down (which very annoyingly hide the next few menu entries so you don't know what's next, but can be turned off).
(6) Oh and on the subject of the menus, they go on forever, bring a book to read as you scroll down. (Plus some of the names aren't that intuitive.)
(7) BTW (not firmware related) shooting stills and comparing with my GH4 using the same lens there really wasn't that much of a difference in detail captured, which surprised me. Maybe the GH4 AA filter is really weak?
I'm leaning towards getting one, but I need to see that scaling the full 20MP sensor for video, rather than using a sensor pixel per video pixel (per GH4) doesn't lead to moire/aliasing effects (like Sony tend to) or a lack of sharpness (it quite amuses me that Sony A7rII users slag off the 5DIV for its crop ratio but then often shoot in Super-35 crop mode as using the full sensor is a lot less sharp).