The relatively low power output of FM transmitters sometimes makes it unsuitable for use in some large urban areas because of the number of other radio signals. This is compounded by the fact that strong FM signals can bleed over into neighbouring frequencies making the frequencies unusable with the transmitter. Removing a car's radio antenna has been found to significantly improve transmitter reception.
Some models which connect via ports other than the headphone jack have no means of controlling the volume, which can force the sound to transmit out from the device harshly (causing over modulation, audio distortion and possible radio interference), or too low. In theory a device could use an auto level controller or an audio limiter circuit to overcome this problem although there are few (if any) devices with such a facility available on the market yet.
There are good and bad ones but the crucial thing is signal strength. There is a thread on here about using the 12v socket in the boot so you can get the transmitter as close to the aerial as possible. The next problem is finding a part of the FM band that is relatively free of other transmissions and while that may work for a while as soon as you drive any distance you may then get interference from transmitters that weren't near you when you first tuned in. If you do find the right set up that is free from noise and birdies then the quality is almost as good as an FM radio station but not as good as cd quality. Also bear in mind that the interior of a car is not exactly the best listening enviroment and will often mask any imperfections. Hope that helps.
i have one i use in my van because the cd player does not work (french) it works fine you can go close to radio frequencies without interferance but mine is powered bt 3 aaa s and it does not take long for them to drain then you get interference like wheelie said removing the arial helps , they sound ok but do become tyring after a while , go for a 12v one to save on batteries but dont blow too much cash on one