I purchased the regular length Marmot Cloudbreak 30, rated to 30 degrees Fahrenheit and weighing less than two pounds, after comparing it to three other similarly priced and designed bags, even climbing into all four and zipping them up around me. I bought it to use when backpacking in wet weather, coastal kayaking, and sleeping aboard a sail boat, all situations where my old three-season down bag might get wet and loose its ability to insulate. After two consecutive nights in temperatures down to 30 degrees Fahrenheit, including rain, sleet, and snow, the second night, I found that the Cloudbreak 30 kept me warm enough if I used the bag liner and dressed properly.
I used the new system in more than ten year old Sierra Designs Ultra Flash 2 person, lightweight, three season. That is right, a three season tent! Even though I was in a three season tent with netting all around, sleeping properly dressed and fueled inside a a REACTOP liner inside a Cloudbreak 30 on top of a short SOlite I slept toasty warm.