pandora and slacker have audio control quality issues. slacker does weird stuff with volume leveling and pandora amplifies some tracks to the point that they crackle anything plugged into the device.
i attempted to cancel my xm subscription, but they just keep giving it to me for cheap, so i'll keep sticking around.
I use the Motorola MOTOROKR T505for wireless connectivity in the car. Both the phone and music sound great coming over the car stereo system. Very rarely do you get any FM interference. Definately worth it IMO.