There are a lot of different types of transmitters available to purchase, each differs in performance and quality. As with everything else in life you get the good and the bad. The problem with transmitters is that the bad are very bad and give the other products a poor reputation on the back of theirs.
FM Transmitters by nature are designed with a slight fault, in that they are prone to interference from other signal frequencies. This can manifest in two ways either the transmitter signal is blocked because the other signal is stronger or interference is received over either audio playback, resulting in echo, buzz or both.
The actual quality of the audio being played through the transmitter is generally quite good. Most genuine products such as Griffin iPod fm transmitters , dexim and or logic3 offer audio equivalent to a good radio station (radio 1 for example) however its generally expected if you use a frequency close to a powerful local or national station you will receive a little interference at some point during connection.
The solution to this problem however is simple. Firstly select a signal which is clear of any radio stations, this will reduce the amount of interference heard, secondly if your frequency picks up any in the future simple change frequency. The Griffin iTrip Fm Transmitter for example has 3 pre-set frequency's to change to if you do happen to receive quality issues with the current settings.
FM Transmitters are a good solution if you only want a simple iPod input in your car, If for example you don't need an integrated kit that offers full digital quality, iPod control and text display the Transmitter could be the kit for you.
A small saying that my granddad used to say to me “you get what you pay for” generally rings true in all walks of life. If you buy a cheap transmitter you should expect the performance to be poorer than other kits, from our experience the griffin tend to offer the best all round performance in the iPod FM Transmitter world.