Due to the size and material used on the brakes of these vehicles, the brakes may start to squeak around the 50 percent wear point. The brake rotor surfaces become uneven, causing a lip to form at the outer edge of the rotor. Because of this, the rotors are usually replaced when the pads are worn (pad life varies depending on driving style and terrain).
Resources for the Repair and Maintenance of your 2011 Mercedes-Benz E550 4MATIC
2011 Mercedes-Benz E550 4MATIC
2011 Mercedes-Benz E550 4MATIC Problems
2011 Mercedes-Benz E550 4MATIC Questions and Answers
2011 Mercedes-Benz E550 4MATIC Reviews
Since purchasing new in July 2011, I have had replaced under warranty the air shocks, front struts and, as soon as the parts arrive from Germany, the interior wood trim. In addition, I have had to replace the brakes/rotors at ~29,000 miles. Shame, really, because other than the suspension and brake issues, I really like the car. The engine is terrific - exactly what a normally aspirated V8 should sound, respond and drive like. The transmission is balky sometimes but, overall, works well. Not sure if the suspension problems are systemic or if I've just had bad luck. Given the frequency of repairs, however, I probably won't own the car long enough to find out.