I've used this on several intercontinental/overnight flights now and it has worked wonders for my movie watching/music listening on the plane: I now use my own Sennheiser earphones rather than the awful airways' earphones. My wife has independently bought this for herself after testing mine on a recent flight (she wasn't sure she'd need it initially, but changed her mind after using it to watch a movie).
The only thing that occasionally frustrates me (as a biggish - 6'2" guy) is that if my thigh knocks against the adapter, then it may interfere with the sound quality. This has been the case with the airlines' headphones before though, so the situation hasn't worsened and I suspect it's more to do with the poor quality of the airlines' seat plug than the adapter itself.
The adapter also arrived very promptly. As with another reviewer I order it one day and it arrived the next even though I selected 'super saver' delivery. I was pleasantly surprised.