Did HKG-LHR on 254 the other day. Seat 40H at the front of economy, aisle, by the angled entry into new PE.
Not bad. Was dreading it but the recline was reasonable; no aching from a compressed lower spine as in the old hardshell slide cusion seats. Bit narrow and fixed armrests as usual in a front row; decent small screen on arm at left while J and K have small screens fixed on the bulkhead. Rather distracting position. Improved VOD.
Plenty of legroom in that row. Could put my bag in front of me once in the cruise and put feet up.
I slept about 8+ hours which is a good sign too. Bettered only in economy by 4 across, really.
So I'd say that if you are paying for yourself and can't get your miles upgrade, the new economy seats in the new and retrofitted 777s are an ok way to go.
BTW, crew were, as usual, v nice to me as a DM. Happy to let me ignore food service and other bothersome intrusions.
Hope this is helpful.