I'll say it again:
If you hike in bear country please do NOT use your jacket, hat, sleeping bag, socks, etc as a cozy. Use something dedicated and small that you can put in your bear bag, canister or Ursack at night!!!!
And yes, cozies DO work. If you hike at altitude, in colder temps and/or use items that require longer rehydration times (think dried versus freeze-dried vegetables or say cooked and dried beans) a cozy becomes very important. You don't really need a cozy in August for say mashed potatoes, ramen or couscous - those all rehdyrate in a couple minutes.