Cannot live without
I use this 95% of the time I'm sleeping in my sleeping bag. It just makes it so much more comfortable. The fabric feels amazing to the skin, and you can wash it when you're done and save your sleeping bag from getting too funky. It probably adds 5-10 degrees of warmth to your sleeping bags rating, but I still like to use it even when unnecessary just because its so comfy. It packs down tiny and weighs almost nothing. I never leave a trailhead without it.