Ouch - I went through this as well with the 300E. I had used the Pep Boys lifetime pads on other cars with no problems. On the MBZ I had big problems.
The rear pads actually made contact with the "hat" part of the rear rotor. I'm talking about the metal plate that the friction material is bonded to. It did this on both sides. The grinding noise was awful. I took it apart and ground in some clearance with a bench grinder as a temporary fix. That's unacceptable quality in a product as critical as brake components.
The fronts squealed like crazy.
To make a long story short, those pads are now in a landfill somewhere. I put Balo rotors and Textar pads and it was the best money I ever spent on the car.
Don't mean to offend anyone, but in my experience the Pep Boys brakes are crap.
Experience is something you don't get until just after you need it.