i've seen them and i've seen them fail. part of the problem with them failing is that they are spec'ed with a clamp voltage too close to the power supply voltage they are being used with. the same problem exists with cheap AC surge strips using cheap MOV's rated only 5 to 10% higher than the AC line voltage. this operates both TVS diodes and MOVs with a significant amount of leakage current, because they are operating at the edge of their conduction curve. long term heating causes them to fail, as does a small spike on the rail after they are already heated. with TVS diodes, the damage is usually limited to the diode itself, but with MOVs operated in this condition, they are a fire hazard, especially in a plastic outlet strip. TVS diodes and MOVs, if operated as intended, can clamp huge amounts of energy, but if they are pre-heated due to being run "close to the edge", the additional energy they can dissipate is much smaller.