I really wanted a Slick Bag, but I’m too cheap for one, so I found a Mountain Hardwear Lamina Z, I believe, for half the price. It’s a 5F bag and a little heavier but it does the trick.
I agree with Dools that 10F seems to be the sweet spot for a cold weather bag for me, although I’m not out West. I’m a cold sleeper, so I pad the rating by 10F or so for a comfort range. So, I consider a 20F bag to be comfortable at around 30F, with some clothes on. That’s not really getting me into the cold weather. But a 0F is too heavy and warm for the cold weather I find myself in.
So a 10F bag (20F comfort) seems like a good number for me. If it’s warmer I can just open the bag up and lay it over me. I prefer that anyway. If it’s edging towards 20F, I usually have some extra clothes I can put on.
If it’s much colder than that, you’re looking at a full-on deep winter setup.