Thanks Gil, kind of you to say so
The 100-400 is really good, sharp and handles nicely. The extending barrel doesn't really seem to affect balance significantly either which is a plus. So much so that I've now sold the 300 f4
I had the 300 PF (which was used almost exclusively with a 1.4x TC) and 70-200 f2.8 FL previously and it's effectively replaced both of them in my kit bag for the stuff I shoot. The Z 100-400 is considerably sharper than the 300 PF and TC combo, with closer MFD which helps for insects, etc.
I'd say the 500 PF is a bit sharper than the Z 100-400 and resolves really well, so you can crop in really heavily and still retain decent image quality. IMO the 500 PF is an absolute bargain at the current used prices
Edit: the only negative with the 500 PF on a Z mount body (I use a Z8) is that because of the FTZ it doesn't balance quite as nicely as on my old D850