They are a gimmick -- save your money.  At best they do nothing; at worst they restrict airflow and cause lower fuel mileage.

Consumer Reports in one of their most recent ads kind of poo poos on various high promising additives and gadgets. Some additives will help reduce friction in the cylinders, though different mechanics/ experts stand on both sides of the for or against argument.

like littlehorn said save your money,that is like the toronad thing you see on tv,not worth a ding.