Product review (3 stars): Took Some Creative Problem-Solving ...
Wanted to use these on a stacked Whirlpool Duet washer and dryer, which sits on a floor of 6-inch off-white glazed ceramic tiles. Before the dryer arrived, I ordered this set (I know the mfr recommends the set with all four pads sticky-version for a stack, but as I wanted to be able to use these to slide the stack in toward the wall and out for maintenance or cleaning, I decided to try the 2 sticky + 2 slide-able instead). Eagerly I tried the slidey pads on just the back of the washer, and was disappointed to see them peeled off as they crossed the  (~1/4") grout lines. Silent Feet's help desk was aware this could be a problem, but had no surefire solution.
  I decided to try laying in epoxy resin across the grout lines (just 5" strips along the path where the feet would cross), shaping it to be flush with the tiles' surface. Happy to report this worked fine, even with the dryer on top of the washer (400+ lbs!).
  As for sliding the stack, I purchased one of those "as seen on TV" EZ-Move kits, so I only need to temporarily replace the front (sticky) Silent Feet with two of the hard-surface EZ-Moves (their lifter helps me do that) to move it out and back in. Those plus the rear slidey feet do the job.
  Once I solved my special requirement, I can say that these Silent Feet are exceptionally durable, work well and were worth the not-insignificant price. I don't have any "walking" problem with the stack, either.