Hot water in a soda bottle is the primary reason to carry a stove at all in snow country for some of us.
Otherwise we follow a no-cook strategy.
As a side note, instant mashed potatos hold heat even better than water though you would want to have them in a ziplock container rather than your bottle.
Gatorade and aquafina type bottles work fine as hot water bottles but just make sure it's HOT water and not ridiculously "hot to the point of melting the plastic" water.
In practice it is pretty simple to have your stove set up just outside your shelter and ready to go for a quick liter of hot water in the middle of a cold night.
As for fuel weight; if the intention is only to make hot, and not boiling, water you will find that a fraction of an ounce of alcohol or of an esbit tablet will do the trick.
I know there are those folks out there that say they have melted soda bottles using this technique.. Well, don't use boiling water.. Just hot water right?
As previously stated, this is probably best used as a suplement to comfort rather than a replacement for true insulation value.

Hot rock works well for me the few times I needed it. Having a sleep system that is accurately rated is the best solution though.

IMHO:
Nothing holds heat better than water. No matter what you add to it, plain water holds warmth best of all other materials. Heat capacity of water is 1. This is what I remember from school physics...

Gatorade and aquafina type bottles work fine as hot water bottles but just make sure it's HOT water and not ridiculously "hot to the point of melting the plastic" water.
In practice it is pretty simple to have your stove set up just outside your shelter and ready to go for a quick liter of hot water in the middle of a cold night.

No and no
Gatorade and Aquafina bottles do not make even remotely a good hot water bottle and for the reason you mentioned : you can't put boiling water in them.
If you start with hot and not boiling water even inside a "cozy" after 4 hours or so at 40f you will have warm (at best) water not enough to make any difference inside a sleeping bag. Below freezing you will have cold water in an hour or so....
As for heating up water in the middle of the night, well it might be simple in mild weather but it isn't in below freezing weather particularly on snow.
The heat you (and your bag) lose by getting out of your bag and spending the time to start the stove, boil, put that in to the water bottle and go back to bed will be more than what you gain from having that liter of hot water.
Please do keep in mind that bad advice for winter camping can kill.

I have found I get 6 hours of warmth from a 1 quart nalgene bottle full of boiling water stuffed in a wool sock. This I have
done for weeks at a time. I also keep my stove at the head of my bed so if I need more for one of those 14 hour nights, I can
reach out and start the stove out from under my tarp and stay in my bag while the water heats. I use a whisperlight stove and
carry extra fuel just for hot drinks and hot water bottles. On really cold trips I also bring 1 pint bottles and use them in the
morning in my double boots to prewarm them and then tuck the still warm bottles in my parka pockets or the small of my back
till camp is broken down.

Of course you must keep your stove far away from any fabric,

Be careful of spilled fuel as hazard of fire or contact frostbite.

CO is a danger in an enclosed shelter.

There is no comparison using hand warmers vs a quart of boiling water. The hot water bottle will help drive out moisture from
your sleeping bag and clothes too, hence you will be warmer in the long run and your insulation will be more efficient.

Try them and see.

I don't know, David. Grabber Mycoal's body warmers put out an impressive amount of heat for nearly 12 hours. Not bad for 1.2 oz. each.

On really cold trips I also bring 1 pint bottles and use them in the
morning in my double boots to prewarm them and then tuck the still warm bottles in my parka pockets or the small of my back
till camp is broken down.

I'll have to give that idea a try! Putting feet into cold footwear is one of my least favorite things. They eventually warm up but takes forever when very cold. Bonus ... it leaves you with a liter "not yet cold" water to drink while you are moving.

Edited by jcolten on 12/20/2012 16:40:11 MST.

You can get the reusable sodium acetate phase change hand warmers, and they are pretty reliable. They are not terribly hot, though. Being recharged in boiling water makes them good.

--B.G.--