I've got nice winter boots that keep my feet warm and dry while walking in them. On the other hand, after about four miles of biking in single digit temperatures, warm boots aren't enough to stop the cold from setting in, and I've got eight miles to bike from home to work.
I'd been hesitant to put hand warmers in my boots, thinking that the discomfort of cramped toes would outweigh that of frozen ones. However, this winter's weather persuaded me to give them a try.
I'm happy to report that the warmers have helped keep my piggies warm and they still have enough room to wiggle.