Be very careful about when you buy it for a car.
Some think that it locks the car into the condition it was in when it is applied. Cars wear out in GT4, and you must get the Rigidity Refresher Plan to bring them back to as-new condition.
However, if you get the Increase Rigidity option BEFORE you restore the car with the RRP, you lock it into the worn-out state. In other words, always apply the RRP before you get the Increase Rigidity option, unless you get it when the car is brand-new, of course.
I don't know for sure if the above is actually true, but don't take a chance on it.