Over the years I've managed to collect well over a dozen dry bags. Many now have little holes which is understandable given the amount of use they have had. One stands out as the toughest, most durable bag I've ever had. It's a basic Sealline bag. Heavy duty blue material, bought at Taunton Leisure in Swansea in 1997 before a big kayaking trip to Norway. Since that time it's been on dozens of trips and never let me down.
 
The Sealline Boundary Pack 35 is a more sophisticated bag in that it also has a decent set of shoulder straps and a waist belt meaning that you can carry it on your back. It's a 35l model meaning it's big enough to use as a day bag, but not suitable for massive load carrying. The material is comfortingly durable meaning that it isn't a lightweight bag, but I have no doubt that it will be around for many years. It's perfect either for loading up with bits of kit, or for sliding a 30l (or smaller) bag straight into if you are simply wanting to transport one in a dry manner.  The roll-top closure is a standard double plastic bar configuration  with instructions embossed into the black bars to make sure that you roll the correct number of times, and in the right direction. There's then a heavy duty plastic clip to keep it done up, with a plastic D-ring for attaching it to a boat or harness. The shoulder straps are thick enough to offer a reasonable amount of comfort without being too heavy or full of absorbent foam. There's a chest strap and a basic waist belt. All of the straps are removable meaning that you can trat the bag as a basic drybag if necessary.

The 35l bag is the blue one in the image above. It also comes in 70l and 115l. The bags weigh 850g, 1160g and 1530g respectively.
 
This bag retails at around £60 which is an absolute bargain if you consider the consequence of getting your kit wet in the wrong situation.
 
More info on the Sealline website.