Repeating, as I have said under "Speaker Cables".
( I am not as sceptical as I may sound. Only it is often a question that the wonderful things manufacturers claim are so contradictory to basic electric principles that all us EEs must go back to varsities or technicons should they be correct. My subsequent impression : If the manufacturer displays such ignorance of principles, how am I supposed to buy his product with confidence? A CEO of a famous cable manufacturer once wrote to me (I could almost hear the soft confidential tone): "Mysterious things happen when two wires meet ..." Really ... need I go on?)
The main claim-to-fame seems to be the reduction of noise, "purifying" of the signal" etc. With a stretch of the imagination that might be true where the mains supply is noisy. This is certainly not true of all (most?) mains supplies. "Purifiers" are usually effective in such a case - but first determine if you need it!
As for special wires: They can contribute nil if anything to purifying a mains input. This should be done in the device (amplifier) in the first place; I am amazed at the number of even high-end products bringing the mains in 'naked' - not even a capacitor across the transformer primary as was the custom before. Futher LC mains filtering was the custom a decade or more ago. As for claims of wider soundstage, more air, ambience, crispier high notes, improvement involvement, hearing the second vioilinist wheeze ...... no comment.
It is simple to do a blind test - comparison with, say, ripcord (representing the opposite end of the scale). The universal advice: If possible compare under neutral conditions and make your conclusion then. (As before, nobody is advising using flea wire).
PS: What is often a factor is the poor quality of mains connectors. They can corrode in a short time, and can be pulled and replugged occasionally to clean off any layers, etc. Some of the OTC consumer stuff is really atrocious.