Granite Gear is all about ultralight bags. The mouthful of a name AirVent HD Reduction DryBloc bag is a heavy duty drybag with an interesting twist: the bottom is made of eVENT waterproof fabric, so when you roll it up the air is forced out the bottom.
We didn't torture test the HD (i.e. deck bag on a kayak), but our intended application was a waterproof bag to go from sled to pack on mountaineering trips, and for that it definitely does the job. It's easy to use, with a nice plastic arched stay in the roll top that makes it easy to grab. The 100D Cordura high tenacity nylon is pretty burly for such a light material. The extra large only weighs in at 3.5oz, so it pretty much fits the bill.
Our only complaint is that the extra large is still not big enough for our application, mostly in the square profile diameter. It's a 33L bag – about 2,000ci – but the mouth is only about 8×11 inches. The length is OK at 2 feet. All of this makes sense for fitting inside a kayak hull, but what about us dry-land load monsters? Where's our love? Oh well, we'll get by.