There are two distinct designs for these things: those that use the dock connector at the bottom, and those that use the earbud output at the top. The design that uses the dock connector also recharges the iPod, and all things being equal, you'd expect it to provide better sound quality than something using the variable output of the earbud jack. I have one from Monster Cable (iCarPlay or something like that). It lets you select from a variety of frequencies for transmission, which is useful on road trips as you move through different metropolitan areas. The sound is OK but not great -- definitely nowhere near as good as a CD.