I don't use it, and I don't think it would be a big benefit. Here's why: so-called "reflective" thermal barriers act to reduce RADIATIONAL heat loss. That may be significant on the dark side of the International Space Station, but it's not a major mechanism for cooling of the feet in winterboots, silvery inner booties of some double boots notwithstanding. You lose heat by CONDUCTION, so good old-fasioned dense felt or foam insulation are what you need to put between your warm skin and the cold environment. Pressure increases conductive leat loss (as well as decreasing vital blood circulation), so IME adding insulation under the feet is especially critical when wearing crampons.