My brake warning came on at abut 40K, saying I had about 5,000 miles left and to change pads. I thought yeah right!! Owned Chevy trucks for 35 years and never changed pads before 70K. Well, I bought a set of genuine GM pads with the brake sensor to have on hand just in case. When I went to do my scheduled oil change at home, I decided to pull a tire and found that the warning was correct and my pads were down to nothing. (Ridiculous). So, I went ahead and pulled the rear tire to find the rear pads were down to about 30% . Not impressed with quality of pads for sure. To make things worse, I inspected my rotors to find that parts of the top layer of steel were beginning to swell and peel!! I did a complete 4 rotor change out on everything and hoping for better results over the next 40K. As far as the brake wear sensor goes, it is very accurate but can be a nuisance as it can load with dirt/debris and brake dust causing faulty warnings to service brake monitor. You must access the trucks computer after installing your new brake wear sensor. The sensor wears down as your brake pads wear down. I went with Brembo UV treated rotors from RockAuto, had already purchased the OEM GM pads, but maybe should consider another mfg. which I never do.
So, if you get that warning at around 40K about brakes.....You're not crazy and should pull a wheel and have a look!