Sleeping bag(s) and your experiences.

I bought a 15 or 20deg Blue Kazoo in the early 90s. I have used it a ton. Longest was 2.5 continuous months. Still performs 100% and looks practically brand new. It feels wonderful and you can position the down in the tubes to vary the warmth. It feels like the cold side of your pillow on a hot night.

I rely more on my pad and a bag liner than the bag itself to keep me warm. When I'm camping in hot weather, I just use a high-end Thermarest inflatable matt with a sleeping bag liner. Both the liner and the pad each compress down to the size of a soda can. The bag liner is like a thermal body sock that feels like sleeping under a soft sheet. When it gets cold, I add a synthetic down, light, sleeping bag. The brand is North Face but I think it's a model they no longer make. The light bag, pad, liner, and some thermal underwear, will keep me toasty down to 20F. Small tents also help. My wife never tent camps with me so I prefer low tents that keep the heat around me. Dropping in some hand warmers also helps.

No bag can save you from sleeping on uninsulated ground or air mattress. I once had a hellish night, as in Dante's lowest level of ice-cold hell, when I tried to sleep on an air mattress without an insulating layer between my bag and the air matress. I was literally sleeping on an ice cube as temps got into the 20s. I had to get up in the middle of the night and get a fire going. I don't remember ever sleeping that night.

The last time I was in AK, I used 2 cheap bags, one inside the other, which seemed to me a lot warmer than a single high end bag (and a lot cheaper). Like others have said, staying off the ground with some insulation under you goes a long way. My setup in colder weather is a cot, egg crate insulator mattress, and one bag inside the other.

I had an old double bag and a single bag (both cheap from sears) that worked well when using the single in the double. However, on an especially cold hunt (high teens), I survived, though I sure wasn’t comfortable.