I tried a set, for a tenner you haven't got much to lose. They did make a significant difference, but in the end I found it to be worse. I can't define their effect technically but they seemed to 'smooth' out the sound, initially even semed to improve it. On my CD player I found the music 'vague', particularly the snare drum seemed to lose definition. I transferred them to my DVD player and again initially liked it, but after watching a film realised the 'vagueness' was there again, some complicated dialect was hard to follow. I also used them on my PVR, and although I don't notice that much of a difference, they have definitely isolated the noise from the hard disc. I also had a go at mounting my speakers on shelves and they were better than without, but nowhere near as good as firm stands.
I guess if you don't have your equipment on proprietary stands (say a book case) they would help, sort of like a high end 1/2 sqaush ball.
Like I said, this isn't a technical appraisal, just my experience of these babies, and they now reside under my PVR.