I think it's the weight of the car (and the tire choice) for the wear but the sidewall problems are somewhat common with Pirelli's and usually difficult to get Pirelli to honor the warranty (they like to blame the "broken bead" problem on the tire installer). My step-mom has an 06 S500 and she drives like a granny, her rear tires wear out every 7K miles (they are OE Bridgestone 18"). I really think the Pirelli is probably the worst tire choice for a heavy car and as much as I prefer other tires over a Michelin but for the S-class I'd buy Michelin PS2 if they are available in the correct sizes.
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