Well, that bag is rated at 0 to -3 (Centigrade) I figure the first night we were out on that trip it hit about -5 and windier than all heck, icicles all over the waterfall we were next to and running water frozen, with the ground frozen hard. We were both fine but a little bit cool - that's about the limit of how low I would normally go with that gear. But it is
definately what I would call "three season" in my rocky mountains.

Now having said that about all I would change for a bit colder colder (let's say -10C at night) would be to add a pound for a bigger fluffier bag, and another pound for a 300 wt fleece, so that would take me up around 11. Put in serious snow and then I'm probably taking a lot more though. I really don't travel light in wintertime.