Actually, I've done it in upper teens/20's lots of times in Colorado . But two weeks ago, my Colorado camp buddy and me wanted to give them a test when we actually knew the temperature. I should say that we switched to MSS bags many years ago and they have treated us well. One time- we left base camp for a 11-hour hike and hunt to Mt. Baldy. Due the the long hike-(uphill- actually up mountain) the whole way- we left the intermediate black bag out and took only our green patrol bag and body to save weight. By the time we got there, snow was 3+ feet deep in the low spots, so we had to walk the windswept ridges to stay out of deep snow. No idea what the temp was, but it was COLD up there. We about froze. So, two weeks ago our forecast here was light snow and 4* with windchills of -4*. In C/S he had air temp of -7 on his phone so he beat me. We messaged each other and took our bags outside and slept. I had on long John's, blue jeans and a denim shirt. I slept extremely warm all night. Got up at 6 to pee outside and checked my phone. 3*. Texted Jeff and he said he went in to pee at 3:30 and stayed in. (He lives in the city so he can't let it fly like I can on the middle of nowhere). Using both bags and the bivy, I was warm and dry when I got up. Not like our hike to Baldy that time lol