probably better than nothing, but in all honesty I think I'd take a wet wool sweater or fleece with a silponcho on the outside over that.. (I'd also happily take a dry inner layer).

It certainly wouldn't do a darn thing even inflated, with my car broken down in 40 below on the QE2 in the middle of bible belt alberta I think I'll keep my silponcho and insulating layer in my pack.. and my MEC parka, sleeping bag, svea stove, pants, mitts, and feltpacks in the car in winter. (I really do keep all that crap in the car then.. only just put the parka away today!)