Having played with quite a few of these little transmitters i can say these are probably the best of its type, if you are after a micro transmitter get one, for a fiver you cant go wrong, I paid £30 for mine a year or two ago, other cheaper units have all sorts of other problems, the itrip is ok if you have deep pockets and a s**tpod but their performance as a transmitter is worse (it has a shorter aerial and uses the same chip - it still has to conform to the same rules )
These little boxes are great but you have to work within there capabilities, The law restricts these devices to a very low power output and short antenna so you will get interference from other devices in the car, fit a suppressor and try positioning the transmitter in different locations for best results, getting an audio extension cable will help with this.
The sound quality wont be as good as radio1, They use very expensive spectral enhancement technology to get a louder, puncher sounding sound from the limited quality of fm, you can forget about competing with that. Set your bass and treble settings to flat (mid way) and switch off any boost or sound profile on your mp3/cd player to avoid distortion, use the tone controls on your car stereo.
The biggest improvement is gained by fitting a larger antenna I can now get a reasonable stereo signal at 50m and mono at a couple of hundred, search the web for "tunecast mod" its a simple case of replacing the 3inch antenna with one of a meter or so (search for "antenna calculator" for the exact length for the frequency you want to use)
The other important thing to note is that it will be interfered with by other stations so choose your frequency carefully as far from other stations as you can, be prepared to re-tune if you travel any distance as the radio spectrum is jam packed and stations are staggered from area to area to stop them interfering with each other.
A better solution for the car if you have a tape deck in the car is a cassette adapter but again these are limited by the tape deck quality, if your looking for a perfect solution the only option is to replace the deck in the car with one that will play mp3's or has a line input for your player....