It isn't as good as a real cover, but it there isn't a lot of wind then it can work to reduce evaporation and therefore heat loss. The problem is that a breeze or wind will push the one-molecule thick layer towards one end of the pool exposing a lot of water. So I wouldn't call this snake oil in that it isn't completely bogus, but it's not nearly as effective as a real cover and needs calm conditions. One of the original products, HeatSavr, uses cetyl alcohol as its primary ingredient (along with dispersant such as isopropyl alcohol along with some calcium hydroxide). There's a long thread about this on another forum
here.