Live in a city where there are no channels without someone broadcasting on it. I primarily listen to audiobooks so any static is noticeable.
Have the Griffin one myself. It works fine and will override the over the air signal by selecting one of the really weak stations. The drawback is the adjustable arm the ipod sits in. Really depends on where your outlet is located in the vehicle. It's location is OK in my truck, but bites in the car. You will not want to have the arm set out to the side supporting the weight of the ipod, or when you go over bumps, it will move and end up turning itself off.
If had to do it over again, would go with a style similar to the
Kensington to alleviate this problem. Would also be able to then hold it in your hand instead of having to reach under the steering wheel or around the shifter and away from the road.
And naturally, if you have an aux input, that is the way to go instead of a transmitter.