On the third time I used this product, the heaters overheated when used for ski boots - one of the plastic risers actually melted (see photo).  The product was so hot I could not touch it and there was a horrible plastic smell.  Fortunately my ski boots did not appear harmed (nor the custom insoles), but I would never trust this product!  I bought two of these and wish I could return, but the failure occurred just past the day of return.