That's a very broad question. It depends what type of head unit (CD player) you have in your car. Is it stock or aftermarket? If it's stock, many of them have special aux adapters you can use in place of where a cd changer would go. The same goes for aftermarket. I recommend Crutchfield.com. They're good with specific vehicles and systems. If you have a really old car/head unit you might just consider a new one that does accept aux. You can get a really decent one for even as low as $150.
This is true, but only good if you live somewhere that is not to congested. I live in the NYC metro area and about 99.9 of the stations are used up already. You can't find an empty station. Another thing is that the music sounds garbage. I used to love my Belkin Car Kit. It would charge my iPod and transmit the music through my AUX input. FCor some reason, it will not transmit the music from my iPhone. I hope they make a new iPhone compatible one.