I have very little trust in the brake pad wear sensor system. When our X5 was still young, and within the initial 4 year maintenance period, the passenger front side started making terrible noise one day, and when we took it to the BMW dealer, they said the pad was completely shot.

I realize the front pad sensor is on the driver side, but the fact that the passenger side was making that much noise and the driver side sensor hadn't fired a warning light, left me with the impression that particular sub-system was a poor design, and could not be relied upon.