We had someone on our flight to Malaga a few days ago who was using one. The cabin crew noticed it and brought it up front. We agreed that it couldn't really do any harm as it doesn't transmit and thus won't be expected to interfere any more than a discman or walkman would.
However, we also agreed that the guy should've asked us before using it. We called him up front and in the end gave him a jumpseat ride for the descent&approach. So there may be a good reason to take it anyway!
