The thing that amazes me about hypothermia is how quickly and thoroughly debilitating it can be. Considering fighting hypothermia in yourself, requires simplicity. Once prevention has failed, anything needing fine motor skills or much thinking may be far too difficult, wasting precious time. (Watch BG try to build a simple shelter and fire in his new Norway episode for a great example.)

That doesn't mean you shouldn't take things like MRE heaters. Just don't expect them to be useful until you're already warming up using the other methods mentioned in this thread. (i.e. get dry, into warm cothes, into a shelter, eat and drink high fat/sugary foods, etc.)