My problem with these adapters (as well as the DIY advice for un-grounded receptacle replacement) is that even if you follow the directions explicitly and check to see if you have voltage to ground at your box, that does nothing to guarantee that you have a low impedance ground capable of tripping a breaker.
It only takes a couple of milliamps of current flow to allow a tester or Wiggy to read properly, but that ground path has to effectively sink at least 15A to trip a breaker in the event of a fault. Many older houses wired with BX will seem like they have a grounded box, but without a bonding wire in that BX it will heat up like a light-bulb filament in the event of a ground-fault and the breaker might never trip.
They've turned what was "just" an electrocution hazard into an electrocution and fire hazard.
-John
