I have used the Seal Line bags on motorcycles, and they are truly waterproof. I haven't used mine on the bicycle because I have smaller waterproof bags that usually suffice for commuting. If I ever get around to a cross-country ride, I will take at least one, though.
A cool trick (courtesy of the Aerostich Riderwearhouse catalogue and probably other sources) is that when camping, you can use them to wash clothes: insert clothes, soap and water, squeeze out the air and seal the bag, then stand on it and move your feet gently up and down. Do a couple rinse cycles. Hang clothes on clothesline to dry.