I had the stephens gel eze black pad with blue gel pads at the front and back (like little circles) Mine slipped like mad! i always felt like it wasnt in the right place. i used it with a half sheapskin pad.
They are really good thought think its a matter of trying them out. i use a normal gel pad now.
I used one a few years ago and was really impressed with it! I found the synthetic saddle I was using on a mare kept slipping, even though it was expertly fitted The Gel eze pad soon remedied this and the saddle stayed put!