Q on play your music source through fm radio thing

i've seen this thing in fry's electronics it's like a small palm size audio gizmo you plug it into your music player and you can listen to it through fm radio in your car or your component system that doesn't have an mp3 playback like me. it seemed like a neat idea for my car i was wondering if anybody ever used one of these and how their sound quality is like compared to car jacks.

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