If courtney reckons modern aircraft never lose the autopilot he's never done an airbus sim... or an embraer sim... or a 146 sim.
Straight autopilot failure: very rare indeed (although turbulence could make the embraer autopilot drop out, especially wake turbulence- it didn't like excessive roll rates)
System failure that makes the autopilot unusable; pretty common, whether it's an aircraft failure (gotta love those hydraulics failures) or a ground equipment failure. Anyone here managed a year of airline operation without going anywhere without a working ILS?