Product review (4 stars): Reduces the vibes, won't eliminate them...
Two sets to keep the second floor washer and dryer heights even.  It does cut down on the washer spin cycle vibrations transmitted into the floor, but the washer actually moves about even more.  Leave more room between the washer and the surrounding objects, otherwise there will be lots of unpleasant banging going on.  Not able to completely absorb all the vibes, but does a pretty good job.