Down jackets can be very light. Mine /is an Eddie Bauer and weighs about 11oz total. The down jackets bring about a small cascade of events in your pack.
By buying one I found it immediatly apparent thet you can use it for sleeping. So, I got a lighter 40F bag and saved several ounces between the two of them, with no loss of comfort. (I seem to remember about 3-4 ounces on the sleeping bag.)
Having one on sleeping, means I get up without getting cold at night. It is just about right for balancing between no exertion (sleeping) in the bag, and walking/moving about camp in the dark after getting out of the sleeping bag.
I try to keep it dry, so, it packs up after breakfast, with my sleeping bag. I really don't notice the volume.
I *have* been caught in snow, and/or cold weather. I avoid winter conditions, generally, but three or four years ago I was out in October and the temp went down to about 10F one night. I needed to wear it for a good part of the next day...till the weather front moved through. So, it also acts as "emergency" gear, along with sleeping long johns and socks as mittens.
The jacket does double duty, generally. Extending the range of my bag (5-10F), and, as camp clothing, reducing the amount of clothing I need (a camp jacket 16-18oz was used around camp.) I am generally as warm or warmer sleeping and in camp, so my comfort was higher. In warmer conditions, I don't *need* to wear it so my bag is cooler to sleep in, it increases the temp range I am comfortable at. I wear it as needed, but this is more emergency use which I did not have at all. Overall, it reduced my pack weight by about a half pound (between the savings on the bag and dropping the older jacket,) comfortably. Well worth it in my estimation.
The down side is down is not real great wet. But, I always have a rain jacket, anyway. Water and clothing is something that becomes more important to think of, if you get one. And, if you do get one, you need to get another lighter bag to realize any weight savings. This can be expensive.