I bought these as I have Raynaud's Syndrome in the right foot and it was becoming unbearable in the cold weather. The thick socks, etc, etc, were not helping at all. I had tried a few of the gel handwarmers to help out to bring the blood flow back, but when I saw these I ordered straight away.
I have been using them for two months and think they are ACE. As a tall, medium-built 36-year-old I can tell you that they can be used safely in shoes as you walk about without them bursting. The only slight downside is unless you get the 'when-to-click-and-when-to-put-into-your-shoe' right then they can be uncomfortable for a few minutes until your weight distributes the gel into an acceptable shape. However, with preventing the outcomes of Reynaud's as the main reason - I don't care.
They provide just under an hour of central heating each (I use both for a whole football match on the terraces - one per half) and have proved to be an invaluable aid. I can now prevent the necrosis by getting these into action quickly - great product.