Got mine last week, love it.
basically it grips really well, so well that if the ladder did ever move (THIS IS MY OPINION ONLY ) I do not think it would travel very far or very fast.
obviously fantastic on grass, but I use mine on everything including the concrete.
Got my mate top stand on it on his drive, I gave it a good kick and yes, it did move about 1 inch. So it is not glue, it can move, it is just it does a very good job of not wanting to go anywhere.