The factory radiator for the E60 M5 and E6X M6 are pretty nice all aluminum units. While they have proven to be fairly robust, age and additional power tend to push them over the edge. Owners who have superchargers tend to have the most issues with their radiators. Between the extra heat produced with forced induction and the heat exchanger mounting in front of the radiator, the factory radiator is working overtime. The E6X models are also getting close to 20 years old and entering the "future classic" stage of life with many owners doing restorations and updates. Anyone who has restored/updated an older car knows the value of having upgraded parts that are as efficient as possible.