As reported last year, I bought a Belkin Auto Kit for my iPod. It contained a cigarette lighter adaptor and a line out from the dock connector, as well as a built-in pre-amp.
I went through two of them, both exhibited the same behavior – Because of the pre-amp, they coupled noise from the engine into the line out signal. Also, the pre-amp was lame and would often whine and hiss even with no iPod connected.
The Monster works perfectly. The audio connector comes off the dock connector, so it does jut out a little into my shifting space but is not bad. The sound is incredible, just clean line out quality, no amplification, pure bass, etc.
The Monster dock connector is much thicker, so I had to modify my iPod Mount by making the dock connector slot bigger. Then I could not disconnect the connector once attached to the iPod due to the space taken up by my holder, so the solution, which I like better anyway, is to push the connector up through the iPod mount and connect/disconnect it there. Then the cable rests nicely in the iPod mount when not in use.
The Monster is much lower profile and I think people will like that also.
The Monster does not pause the iPod when power is lost – This is fine by me, as it is one less capacitor in the chain to screw things up.