I had the Premium horn on my Sportster. It was GREAT! The factory horns on the Sporties are barely usable and no one hears them. I do have to agree that the chromed plastic was kinda chinsy. And it did vibrate like crazy. But I never had any issues with it.
I have the HD premium horn and it is a lot louder than stock. I looks better than the cowbell. If I had to do it over again I would go with the metal arts or other that looks like a trumpet, they look cool.