Mrs. NM gave the QF 789 a thumbs-up last week when she scored it on a MEL-BNE sector. But she was in seat 2A so I expect that made a bit of a difference. I was surprised to find it on the MEL-BNE route, and only realized when doing seat assignment for her and saw the layout. When booking the flight I just assumed all BNE-MEL-BNE were operated by 738 aircraft.
While I have not been on a 789, I would imaging the 3-3-3 config is not much different to a 3-4-3 Y config on 747 or A380. Perhaps the seats are 1/2 inch narrower. But a sardine tin is a sardine tin. The A380 has the possibility of upper deck Y cabin seating in 2-4-2, so that is a bit of an advantage. And 747 probably feels more spacious due to higher ceiling.
Would be book a 789 flight for a Y BNE-HKG flight? Well it would depend on the options available to me at time. Schedule is always more important to me than aircraft type. It would also depend on with whom I am travelling. If 2 people, then perhaps a QF A330 2-4-2 might get my vote ahead of 789 3-3-3, but if three people, maybe 3-3-3 is not so bad, especially if its a full cabin.
And the only way I getting on any aircraft non-stop LHR-PER, SYD-JFK is in the J cabin. Otherwise a connection/stop is always going to be my choice. I am not so time-poor that I need to endure 17+ hours on any Y cabin. 14 hours trans-pacific is bad enough.