I use the chemical warm packs too and like them, but one thing to note is that they need oxygen to function properly. In a watertight boot they can run low on oxygen after 20-30 minutes and start to cool off. They aren't shot, you just have to take them out of the boot and give them a good shake in the fresh air to get them warmed up again.
You can use this to your advantage if you want to use them on short commutes or other quick rides. Just carry a good double seal Ziplock freezer bag with you. When you complete your short ride, take your warm packs and put them in the Ziplock and squeeze out as much air as possible before zipping it shut. Within 15-20 minutes they will start to cool off. About 10 minutes before it is time to go outside again, open the bag and give each warm pack a good shake in the fresh air. Presto, toasty warm fingers and toes without opening a new pack.
"Those who say it can't be done should stand aside for those who are doing it."