Yup, the control box going on a 3100 sounds plausable alright
RE: the CK3000, the voice dialling is a feature of your phone more than a feature of the kit. Even the basic Nokia CK7W will do this. It's a very handy feature to have.
Renaults/Citroens/Pugs tend to pause a cassette or CD when either the mute button is pressed, volume is turned all the way down, or the mute function is activated via the mute wire from the phone kit.
The ones with the colour screens are the biz, as Russman said, the extra features like the audio-in would be worth the price of the kit alone to some people.