You won't regret getting this bag. I put my backcountry bed in a 13L and it's great. Small and waterproof, it's cool how you can squeeze the air out when the bag is rolled and sealed. Awesome find, bags offered by companies other than GG don't hold a bag to them. Wish I could put my backpack in it.
This does exactly what it says - compresses and keeps your stuff dry. I use it for my sleeping bag and down vest. It compresses more than the original bag, weighs less, and adds water protection. I haven't tested it in full submersion for an extended period, but it certainly copes with spills and general moisture.