I used to work at a car audio store. I've installed literal hundreds of them. I guess it's better than nothing; but I would almost rather sing to myself. The only decent ones are the ones that physically plug inline with your antenna. The broadcast style are so limited in power that they don't work very well with the crowded local FM band (I swear some of these stations are broadcasting way above their limit)

I guess I am probably pickier than most when it comes to sound quality.