Thank you, everyone. Jakov, you're always so kind.
I've been waiting for a lens like that for decades: 500mm or above, good quality and handholdable. I've tried the 600/4 and the 500/4 but I'm often in woods and these are too long, bulky, heavy, etc ... to get out of the path. I had a 200-500 on loan for 3 months last year and I found it not so easy to manoeuvre. I was frequently late on the action with birds. I'm often using the 600/5.6 AI on a monopod these days so when FNAC had the PF at less than 3500euro for a day, I jumped on it.
I think it is indeed worth it. I've only used it for a few hours but I definitely like the images and its rather low inertia hand held. The VR seems good enough for me, at least on the D750. Still need to use it on a DX though.