This foot warmer is a boon. I have used it regularly since I bought it, warming up my feet as I sit at the computer. As other reviewers have said, it is not a strong heat. This did worry me initially, but I don't think it matters. Although the temperature, even on 3, is low, it warms my feet up without a problem, so they no longer feel uncomfortable. Which is what I wanted the foot warmer for. I don't need them to be any hotter (indeed, if they were I am not sure it would be good for them). The heat is pretty much what I would expect at 100 watts; in any case, a higher wattage would increase the running costs.
That said, with a low temperature on the highest setting (3), I'm not sure why there is a need for, or why anyone would want to use, 1 or 2 (I haven't bothered to try them myself).
As far as potential for the lining coming loose (which has also been commented on), this tends to happen if you pick the muff up by the back; there is no problem if you pick it up by the front. There is a certain amount of looseness in it but I don't find that a problem; it is after all a removable lining.