They do a nice job of slightly blocking the opening of the intake and then adding a tiny bit of turbulance to the incoming air. The first is a negative (especially in the cars with an intake larger than the diameter of the piece and therefore requiring the additional clamping mentioned in the instructions) and the second means absolutely nothing as far as power and/or fuel economy go.

Spend the $89.99 on gas.

Or oil.

Or flowers for your wife or girlfriend (but not both) on Valentine's Day.