Al is right. Some physiologists estimate that up to 30% of heat loss is from respiration. (See 2nd paragraph under "Physiology" at http://www.emedicine.com/emerg/TOPIC279.HTM.)
The Himalayan Rescue Association (an organization I once worked for) seems to recommend devices like this to prevent "Khumbu cough" and such devices seem to be more common among high altitude climbers and trekkers. (I wasn't aware of them when in the Himalayas in '87)
I just acquired a Psolar EX face mask (39 gms or 1 3/8 oz size large/XL). It is surprisingly easy to breath through. It is too warm at sea level here in Bellingham right now to really test. Hope to get up in the Cascades to give it a try soon.
Had I seen this product first I might have gotten it instead or might try it against the psolar. A little more expensive and strange looking but half the weight at 20 gms. http://www.humanedgetech.com/shop/product.php?productid=177&cat=17&page=1
I have read that these devices work better as it gets colder and at increased altitude. 20-39 gms to stay/sleep warmer, loose less body water, and have less condensation around your bag/quilt/shelter seems too good to be true.