I use the chemical warmers for both hands and feet sometimes, when it's in the 30's. It kind of depends on the day. We buy them in large quantities from a hunting supply store, for x country skiing, where it's often hovering around zero when we go out. I have actually used them with my winter shoes, when it was in the very low 30's and windy, as well as with my regular road shoes, along with wool socks, toe covers and neoprene booties. Can you tell I don't want cold feet?
I second the advice about making sure your shoes are not too tight. I had a miserable ride last week because I was wearing my woolie-bully socks with my winter shoes for the first time. Not only were my feet cool, they were also going numb. I had to stop and untie/retie.
I use the chemical warmers more for my hands. I cannot tolerate gloves any thicker than the full fingered ones that are good for down to 40 or so, so I bought a pair in a bigger size and use them with my Smart wool liners. I put the warmers in between the liner and the glove, on the top of my hand. I have a pair of perfectly good AmFibs, but one time I sort of got them caught in the brifter while shifting and I never wore them again on my road bike. I can handle the trigger shifters, though. I also tried the lobster gloves, but it was just too awkward and weird, so I gave them to my son. I have pretty low tolerance for anything different, requiring different muscle memory, so if using the warmers helps me not kill myself, so be it.