I am rated on the 777, 747 and 767 and I must admit that there is a HUGE difference to my overall health following a flight on the 777! My airline recently introduced services from the UK to SYD on the 777 and many of the crew are reporting feeling far more exhausted after the 9 day trip then they did when it was operated on the 747s.
The aircraft air is very noticeably dry in comparison to the 747 and 767. It seems to be that Boeing were perhaps skimping on humidifiers when designing the plane.