I've been a big fan of Griffin Technology's accessories for Apple kit ever since I bought my first iPod. One of the first things I scout out every time I buy a new iPod (or these days, iPhone) is a way to hook it up to my car.
Back in the day, I used their iTrip device to broadcast music from the iPod to my car's stereo via FM. But having to change stations gets old quickly when driving a decent distance. Griffin has mitigated that over the years with new iTrips that scan for open frequencies, but I hopped ship to AUX ports when those became popular.
My all time favorite car accessory was the TuneFlex AUX with Smartclick for my old iPhone 3G. The best part was the remote that attached to the steering wheel. Fantastic device. Unfortunately, that doesn't work with my iPhone 4.
So I tried their new TuneFlex AUX Handsfree. That didn't work out so well.
Finally, I've settled on a simpler solution. The iTrip DualConnect. It can connect via either FM or AUX. I took the AUX road. It charges your iPod or iPhone and acquires the audio as line-out via the Dock connector. I'm a big fan of line-out.
The play/pause, forward, backward controls are nice (but I do miss my steering wheel remote).
All in all, it works as advertised, unlike the TuneFlex AUX Handsfree. Consider it highly reccommended.