I bought this because the volume from my iPhone's headphone socket was low, when played through the "aux" input on my BMW Professional head unit. I was forever increasing and decreasing the volume control when changing sources between CD, aux and radio. No more! Simply plug the adaptor into the dock port of the iPhone/Pod/Pad, connect the other end of the cable to the "aux" port and enjoy line-level output, same as the CD input - perfect.
Only blot is that the cable could do with being a couple of inches longer (who couldn't?). Still, at this price, you really can't quibble - an extension cable would only be a few quid.