I received a couple as gifts but I also see these quite often in tkmaxx. Mine came with a thermometer and guide as to which ones to use, I was given a regular one and a travel one -both light weight/tog I just can't remember the number.
I think the travel one is designed to let you put them straight to bed if they have fallen asleep in the car.
Good thread - was wondering about the sleeping bag type things myself. I have a friend with 2 kids who swears by them!