Not very sticky.
I have tried these on multiple types of flats to help fill up a bit of room in the back.  They do not stick very well.  They held on to certain surfaces for a few hours of wear - at most.  I ended up using super glue to stick these into my shoes, which worked great.  The surface and thickness of this product definitely helped keep my shoe on and was comfortable.  However, the shape isn't ideal.  For the back of a size 5.5 flat shoe, the angle seems odd and it's difficult to align it so that it's flat against the shoe.  I would recommend spending a few extra dollars for a better product.
Rating: 2 stars