I use Snugglesafe for the abandoned cat that lives on my porch. It works ok, but like someone said, it does get very, very hot. I use a potholder to remove it from the microwave and tuck it under an old sweatshirt that the kitty sleeps on in a box that we made for her. My father has plug-in warmers that he uses for his 2 strays that live by him, but you have to be careful that they do not get electrocuted.
Snugglesafe was worth it for me. I just have to keep my eye on it when the temperature gets below 0 at night in the winter, and warm it more often.