Just had notification that I should receive mine next week (in the UK).
I've thought long and hard about this lens, as I already have the 400/4.5 and the 800PF - and have been more than happy with the results from the 400/4.5 with the 1.4x TC.
My reasoning (ok, clutching at straws to validate the purchase) is that the 600PF 'should' be a touch sharper and quicker than the 400/4.5 + TC (although hard to imagine that this would be a massive difference in the real world)... but I'll have a decent 600mm (that if you'd asked me what my dream Z lens announcement would be a year or two ago, this would have been it, albeit I'd have preferred it at f5.6) and I'll have a faster 400mm to use in lower light.
Tbh, the 800PF hasn't seen much use since I got it (when it first came out)... I don't photograph an awful lot of birds, so my usage for that hasn't been as high as I thought it would be... and that's the one that might be at risk. Ok, who am I kidding, I'm bound to keep all of them!