This is a simple product that is constructed well and easy to use.  Cost is low.  I don't recommend forgetting the chocks are in place and pulling the trailer over them, as these lightweight units tend to squish a bit when run over, but they are a good tradeoff of cost and weight versus holding ability.  I haven't had any problem with them slipping on a variety of surfaces including blacktop, gravel, dirt, grass and cement.