Did this last year for LAX-SYD. Found it to be surprisingly comfortable even in Y on the A380 (though I did pay a small fee to grab a bulkhead seat on the way out and an upper deck seat on the way back), and QF service was good. Wouldn't hesitate to do it again if I were traveling to Australia.

Oh, pro tip for anyone east of the Rockies: the FA noted that even the A380 is generally weight-limited on the DFW-SYD flight, so if you book in the Y cabin on that flight and play your cards right, you may have a decent chance of getting an empty seat next to you.