i want to get one for my parents. i have one but they dont sell them anymore. plus i have read on here about using cables on ios4 that are not apples cause the
"This accessory is not optimized for this iPhone..." (of whatever the exact wording is)
my local apple store said "In those situations you would use a headphone to headphone cable"
Don't count on the Line Level audio being superior on most recent iPods, that is actually a volume tap and you would not have enough of a difference with a car audio system to tell.
Rather than a straight line out with no connection to the amplifier or volume control recent iPods have a volume tap (with the volume control) and can be used to adjust output volume.
Unless you bypass the DAC in the iPod and use a very high end outboard one, like the Wadia, and have your music in AIFF, WAV, or Apple Lossless format you won't hear any difference.