Picked up 2 of these a while back. Haven't had much chance to test them until last weekend. I wanted gear bags that I could put in the roof rack and not worry about (and also are a little more attractive than the black trash bag method). They're a little pricey but well built and survived a trip in a good rain storm. Highly recommended.
http://www.overboardusa.com/classic-waterproof-duffel-bag-130-litres.html
http://www.overboardusa.com/classic-waterproof-duffel-bag-130-litres.html