It depends how waterproof you want it to be!
I have a MSR Seal Line Seebag, 40l, which is very robust, and very good at keeping kit dry, but it is heavy as hell. I think I would rather have a coated fabric bag, over the clear vinyl, I dont see that being see-through is an advantage to me, and it might be tougher/lighter to use fabric backing. Anyway, I have used it when open canoeing, kit sits in a big puddle in the boat bottom. I also have a 13l Ortileb bag that will fit in a side pocket, that too is a bit on the heavy side, but keeps things very dry.
I have friends that really like the Exped bags, very light weight, but I am not sure I would trust one to keep kit dry if it was sitting in water all day, they feel more like insurance for kit in a hiking pack.
I have actually used heavy gauge plastic bags with some success. Both inside and outside (strapped) to packs. The two problems are ensuring you don't poke a hole in them, and doing a really REALLY good job of rolling the top down. I know they won't keep things as dry as a real roll-top bag, so only use them as insurance, or for stuff that isn't so critical.
http://www.backpackgeartest.org/reviews/Stuff Sacks/Dry Bags/
isn't much, but is all I can find quickly
