I have had this particular product for almost a month. It has three levels to keep the coffee warm. On the highest level, the coffee remains lukewarm to warm. However, I should mention that the surface of my cup is not flat. I believe that if it were, it would keep my coffee at an even higher temperature, as the surface gets quite hot.

It has the classic problem that the surface is flat, but the bottom of the cups is NOT! In the end, it keeps the coffee somewhat lukewarm, but it consumes energy (it requires at least a 2A charger and ITS OWN CABLE).
20'' in the microwave is a better solution. In the end, after 4-5 times of using it, it died and its soul transformed into an expensive coaster.