Interesting. . .my REI mummy bag is in the big storage bag that it came in, inside a very large duffel bag that doesn't compress it, in the back of my truck with other camping gear. It's nice to know that if I get stuck on the road I can be comfortably warm regardless of outside temps.

My walking kit/get-home-bag has a lightweight Thermolite Mummy Bag Liner, rather than the bag itself. If I'm somewhere that the bag liner isn't enough, I'll stay with the truck and all the gear that isn't in my walking kit.