[Flubflow]

I suppose its not so bad if you drive around the same area and you use a frequency well away from other radio stations but if you travel round the country a lot then, unless you are aware of the other stations and you keep changing the transmitt frequency accordingly, there is a good chance that it will at some point interfere with other people's radio reception (especially in town/traffic jams where there are more cars close to your transmitter). It also depends on the quality of the unit (i.e. if it drifts and/or emitts harmonics at other frequencies)

A better device would be one that fits inline with your antenna cable. It is extremely low power (its more of an inductance than actual transmitting). The only disadvantage is that it is not very portable from car to car.
I used to have a link to a website that sold such a device but I can't find it at the moment.