I got a Kensington Liquid FM transmiter free with my S9. It's pretty good in as much as these things can be. As others had said it's no match for a cable connection.
Some transmitters only transmit on a certain frequency which can cause problems if you travel through an area where a strong radio station is broadcasting on or near that frequency.
The kensington has a handy feature where it scans the entire band and picks the three most empty frequencies it can find, which can then be chosen from three buttons on the front. It also has an inline USB power socket which you can use to power/charge your device.