With the prevalence of portable CD/MD/MP3 players, the possibilities for this to make money on short flights always had to be very low.
On a sector of over two hours, it might be worth getting one but not on shorter flips. For medium haul (2-5 hrs), there is some chance, as it can make up for the sophisticated systems (free) normal only on long haul.
For myself, I would not bother. I choose the music I take with me as well as the reading matter.