Geoff Francis , Apr 01, 2009; 08:04 p.m.
Don't have any SDM lenses but have plenty of Canon USM lenses, which is the same thing. There are hardly ever any reports of ring USM motor failure in the Canon forum. It make sense as the technology used in these motors is far cleverer and far simpler than traditional arc drive motors.
Based on the design I would expect USM/SDM to suffer from an exceedingly low rate of failure compared to arc drive electrics coupled to a drive shaft.