I'm not sure if it's a common problem or not. The service manager did say that he had another non-AMG that day with the same problem, but this dealership probably does a hundred Mercedes a day (MB of Arlington, VA). My car had 105k miles on it, no mods and not driven hard. BTW, dealer charged me over $400 to replace idler pulley and belt.
Tightener Pulley (part # 000-202-09-19) - $44.46
V-Belt (part # 014-997-71-92) - $73.50
Shop supplies (BS charge) - $41.38
Labor - $275.88
At least I got a free tow out of it