all of the time. my iphone is also my main car navigation device. the port allows me to charge and send line audio to my car stereo system all in one.
Should have mentioned this above as well, but THIS. I installed a Dice Electronics box in my car years ago that allows me to hook up the dock connector in my car to my phone and charge it quickly, feed audio directly into the sound system, allow Siri to work through my steering wheel, and have GPS directions chime in on my car's speakers. It's just an elegant all in one connector.