I use these when kayaking. On the pack it states that they are waterproof but cannot be submersed(?) Either way, water stays out completely and that's the main purpose.
By closing them in a specific way, I can get them to retain quite a bit of air. This makes them super-buoyant; in the medium (8 ltr) bag I had a full flask, ice pack, food, 1/2 ltr bottle of water. I placed it on the water and it floated very high up on the surface, hardly sunk at all, and because of the shape of the bag, the 'open' end is always up and out of the water completely.
If you seal them in a different way, it must be easy to get the air out before closing so that you can get everything nice and compact for putting into a ruck sack. They will keep everything bone dry.
The only reason I don't give them 5 stars is because I'm not sure if they are tough enough to stand up to long term use. They are very light and thin. Time will tell.
UPDATE 25 Apr 2011: They don't get a huge amount of use, but they seem to be holding up very well in terms of seams maintaining integrity, so they still keep things bone dry. Their [very] lightweight materials and construction make them a real joy to have and use.