This jacket is incredibly light. It comes with a stuff sack that seems to be made out of the same material as the jacket. It is feather light. I wore it at camp in the morning and nights, with only a base layer underneath. The temperature was in the mid-to-high 30's and low 40's. I stayed very comfortable. No complaints what so ever from this product. Mont-Bell hit the nail on the head with this one.
About what size is this jacket when it is packed down/stuffed?
I bought two of these, size medium. I used one for my sleeping bag (Marmot Pinnacle) and another for my clothing. This item kept both completely dry in multiple down pours. The construction and materials are superior. Sea to Summit has hit the nail on the head with this item. Keep in mind, however, that is is not 100% submersible. There is no way to completely seal the opening, so it might not be a good choice for activities such as kayaking. But, for backpacking, I don't think you can find a higher quality stuff sack.
Great jacket. The only reason I that I am not giving it a 5 star rating is that I wish it was a little bit lighter and that I could pack it down a little bit smaller. But that is only because I am slowly becoming more and more conscious about weight and space in my pack. However, the construction, materials, quality, etc. are superior on this item. For an everyday rain jacket this item is excellent.
It works great. I have an Osprey Aether 70. The large size worked great. It kept all of my gear dry. It is relatively light weight and packs down fairly small. I was very satisfied with this item. It kept my pack dry through a night of down pouring rain and while hiking during the day. I would definitely recommend this item.