I checked the directions for the MRE heater that reposes in an odd corner of my gear closet. If I were hypothermic, following the instructions would be fairly challenging. There are several fire starting techniques that only involve large motor skills that would probably work better - and getting a fire going has worked fine for me in those times when we were dealing with hypothermia.

What these heaters provide is flameless cooking - a very good feature in a combat zone or in extremely dry conditions, but actually somewhat counter productive in the usual hypothermic situation.

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.)

My kit is made of:
carnation breakfeast package
bivi bag by adventure medical kit ( but maybe will change to Blizzard one)
and a garbage bag
vargo wood stove with some wet fire
bothy bag
down booties
extra sox, buff, gloves
Experimenting right now with MRE heating pad.

http://download.journals.elsevierhealth.com/pdfs/journals/1080-6032/PIIS1080603201706771.pdf

http://download.journals.elsevierhealth.com/pdfs/journals/1080-6032/PIIS108060320970081X.pdf