My GR3 arrived today. Surely love it - very responsive, touchscreen implemented very well (a godsend selecting focus point), new handling tweaks work (the main Fn button's many tricks), and the redesigned lens is uber sharp - with stabilization to boot.
The AF does struggle in low light but as one who think the previous (X-Pro2, X-T2, X100F) generation Fujis focus more than good, I'd say the overall performance is quity decent. Not up to the current standard but certainly a few steps beyond the previous GRs...
The real culprit, as expected, is the battery life. I went through one and a half batteries in the ~3 hours of playing and setting up the custom functions. I've stocked up 5 batteries before hand - looks like I'd need all of them plus a double charger should I be traveling and shooting all day. I assume it's the high-performing processor that's the most power hungry.