There was virtually no activity in this area about 6-7 years ago, and when I first heard of it 4 years ago, it sounded most unlikely. I ended up borrowing one (a Cardas Reference) from my dealer and could not believe the difference it made on my CD player. That cord did not make much of a difference on my integrated amp, but a TG Audio HSR made an amazing improvement.
So, yes - they do work, and what's more frustrating, they are system (component) dependent. If you tell us where you are thinking of using one, someone who owns the same gear as you may have a recommendation.
Buy something used and get your feet wet. Make your own, if you are handy. You won't believe how well they work.
Regards,