I have been an avid hiker and backpacker my entire life. I used the Marmot Helium EQ in all kinds of weather. I've used it in freezing weather, snow, heavy wind, rain, and hot dry weather. During the 2004-2005 winter season I hiked some Presidential Mountains with my friend. I brought minimal clothing but decided to rely on the Marmot Helium EQ for warmth while camped. At night on top of a mountain the temperature reached -20F. The Helium EQ iced on the outside like everthing else but did a good job of keeping my feet dry and warm enough to sleep. This bag is only rated to 15F but because of the high loft it could still retain some warmth in negative temperatures. In 2005 I decided to do a thru-hike of the Appilachian Trail. I had bought the Marmot Helium EQ for this purpose and started out in June of 2005. I started in Maine; it was cold and wet and sometimes still snowy. I was very impressed with the Helium EQ because it was always warm enough and did a very good job of keeping the water out. I used the Helium EQ sleeping bag everyday for about 1.5 months on the A.T. It is impressive because it has a very low weight, the highest goose down available, its waterproof, and a full zipper.