My main bags are a Marmot Hydrogen (30 deg., 1 lb. 7 oz.) inside a Marmot Helium (15 degrees, 2 lbs.) when it's cold. I also have several other bags, including an ancient Raging Inferno. I used to have a Blue Kazoo, but I gave it away to a homeless friend who was down on his luck. It was a great bag. Since I car camp, I don't worry about down getting wet, but if I backpacked, I might carry synthetic.
I don't zip up my bags, but sleep on a memory foam pad, which keeps me warm. I'm a little claustrophobic and don't like feeling trapped. With those bags, I am never cold, but if it gets damp, I have a lightweight Pendelton wool blanket I throw over everything. Each of my dogs also has a down bag, and between all three of them and my Marmots, it's kind of like sleeping in a big nest. Nice and cozy.
If I backpacked, I'd make them carry their own bags. I like sleeping with dogs, and an engineer friend actually figured out how many BTUs each dog burned during the night, and it was kind of like having a little furnace along.