This works great with the XM reciever i use through the AUX input on my car stereo - the difference in quality is unreal!It is NOT needed for MP3 players or IPODS if you have a Ipod car charger with a line out feature that uses the USB out instead of the headphone jack. If you have to use the headphone jack, then this is a good buy.The worst complaint is the battery compartment as it is terribly difficult to open and I am sure I am going to break it (the compartment) some day. AAAA batteries are pretty hard to come by and you have to get them online for a good deal.This is great for satellite radio units and anything where you need to use the headphone jack.