I'm leery of the long term effects of electromagnetic radiation so close to the body.

My best recent purchase has been a Thermalon heating pad. It's stuffed with silica desiccant beads, so it absorbs moisture from the air and releases it to moist heat when you microwave it. Stays warm plenty long enough for me to get to sleep (and I'm ALWAYS freezing, especially at night, and it really interferes with my being able to sleep if I don't have a heating pad). And, it's washable! (Though you have to let it dry 24 hours, which can be a long time to go without a heating pad in winter ... you could always buy two, just like traveling with workout clothes, one to "wear" and one to wash!)