Well. I'd say mostly they work.
Currently, four out of my sixteen chickens are sporting them. They all work except for one, and she was the first and only one to wear them for a long time. They worked on her, a two-year old EE named Flo, for a good eighteen months. Recently, it appears Flo has grown new wiring in her brain, which is much smarter than an average hen's.
Flo can shave feathers with the speed and accuracy of a hen with normal vision, and probably even better. I've had to resort to ordering some Bumper-bits from Britain for her, a device that resembles peepers in that they have prongs like peepers to fit in the beak holes, but the main part is a "bit" that blocks the beak from closing all the way so it can't grasp a feather shaft to yank it out.
The trouble is, the manufacturer doesn't ship to the USA, so I had to order off e-Bay.