I've heard great things about the Thermacell boot liners, but can't justify them, myself. Like others have said, a good liner sock (preferably merino wool) with a good merino wool sock over that with good insulated boots is wonderful.
I absolutely love the liner sock and hunting-specific sock by Smartwool. Out here, when I hunt Dec-January, mornings are sub-freezing and a pair of Smartwool liners with the Smartwool hunt sock (heavy, over-the-calf; also crew) are great inside a pair of insulated boots. When the sun finally rises and the day begins to warm after the cold-lag, I end up having to switch to uninsulated boots, but I keep those socks on (extra padding when you take your boots off to complete a stalk in desert scrub). Smartwool liners and hunting socks are by far the best cold-weather sock combo I've ever hunted in!