I have a Marmot Helium. I have used it comfortably sleeping in the open to temperatures around 20°F; in a tent it is at least 5° warmer. It is a half zip bag. I have never found the half zip to be a problem in warm weather but I also know that many others don't like it. I have used the Helium as a quilt in nighttime temperatures around 50°F and have not been uncomfortable. Of course I am in my 70's and don't sleep as warm as I once did.
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May I walk in beauty.