In reply to afx22:
I've got a Snugpak Sleeper Extreme, or some such macho name, rated to -7oC, and yes it weighs a ton and is bulky as balls (it's from their basecamp series, so isn't designed to be packable) but would be perfect for car camping.
I carried it up Helvellyn on a two-nighter last February and was glad of it and warm in a fleece and that bag down to -12oC at least. Mine has a neck baffle, though I'm not sure about the fancier lightweight models.