I flew the 767-300ER for about 4 and a half years, and it is excellent for long range flights. The flight deck is roomy, but you can still reach everything and there is storage space for all your gear (and duty free shopping) plus two spacious jump seats. The main seats were extremely comfortable but not sleep-inducing because you were able to stretch your legs without hitting the rudder pedals. The longest flight I did was about 12 hours and although tired at the end, not exhausted like a days work on the 737 with it's cramped flight deck, high noise and short sectors.
