I have the adapter.
The reason why is because the adapter is made to be plugged directly into the ipod, not the dock. The connector is the same, I guess you COULD use the dock, but I don't think they designed it thinking people would put the dock in their car.
Anyway, the sound quality is a step up from using the headphone plug anyway, which is nice. THe downside is that since the cases which ship with the ipod don't have a hole for attaching things to the I/O port on the ipod, the best solution I have is to have the case on the ipod upside down. Definitely not ideal. I have a case from
www.sfbags.com coming tomorrow and that will allow me to use the slot on the bottom, keep it in its case, and even have the remote sticking out of the top of the ipod for easy access. That will be much more ideal. I'm still frustrated with not being able to find a way to do a direct line-in into my car stereo.
The Belkin is a nice product. Once you set the amp volume, and attach the cassette adapter to the built-in plug, it's nice to just plug the ipod into the adapter and be good-to-go.