Some are better than others..
I've found the transmitters with Aerielle (manufacturer?) technology work the best. You can find them under the iRiver and Kensington nameplate for about $40. Do a search on Amazon and you'll see the similarities. I used the iRiver transmitter for years and it was acceptable. I especially liked that it plugged into the 12v outlet so I didn't have to carry spare batteries with me. My husband had a Belkin transmitter that was awful beyond comprehension so I'd avoid that one at all costs. Best Buy has a transmitter that looks a lot like the iRiver/Kensington model but I'm not sure if it uses the same technology. If you can't be sure of having a 12v outlet available these transmitters won't work for you, btw.