One thing my fitter told me (he also happens to be Catrin's) is that the charcoal warmers will not work as well (or at all) if you use them in conjunction with full shoe covers. They need air to work. I use them alone or with toe covers and they work pretty well. Any by well, I mean that I can ride semi-comfortably for a limited amount of time. I've yet to find anything (sort of winter shoes) that keep my feet warm indefinitely.