Read The Manual
I recently ordered this item and have only used it twice.  So far, it's good.  According to the manual, it will maintain heat, not heat cold water also it is recommended to only use for 30 minutes.  It advises to fill it with water that is the temperature that you want.  I tested the maintain heat theory twice.  With each use, I eased my feet into hot water.  After about 15 minutes of getting adjusted I took my feet out, dried them off, and put my feet back in about 10 minutes later.  The water was just as hot.  It maintained the heat.  It wasn't that the water was cooling, it was that my body got adjusted to the temperature.  Same as taking a bath.  I haven't used the handheld thingy yet.  When I do, you'll see review number 2.  Enjoy.
Rating: 3 stars