I've got one but bought it on a whim so hardly ever use it. They squish down quite thin (so won't alter fit of saddle) and seem to do the job fine. If you mean one of the 'transparent' ones, will also leave dinky circles all over neddy's back
We use them on a few of the 'rounder' riding school ponies as it stops their saddles from rolling so much, their easy to clean too just stick them in the washing machine and then hang them over something to dry.
I use one to stop my saddle from slipping and it is excellent. I've cut it to fit neatly under saddle and it can hardly be seen. It comes up clean by squidging itunder a cold tap (great fun).
Apparently they are excellent for shock absorption, and are recommended by Robert Smith.