Rboe - I have the same dry bag as the one on the bottom, I think.
http://www.twistedthrottle.com/dryspectm...le-dry-bag
The pluses are that they're completely waterproof, the tie down straps work great, and the bags have a semi-rigid plastic liner that helps the bag keep its shape even when only partially filled. Also, the integral web loops on the top of the bag allow you multiple options for strapping on additional stuff.
The only minus I have found is that the opening is not as large as one might expect. They may have corrected this on later editions because they use the word "wide-mouth" in the description, but they look the same in the pictures. It's not a huge deal and I solved some of the annoyance by packing clothes in smaller bags and sliding the bags into the dry bag. This served the dual-purpose of getting the clothes in the bag easier and keeping them organized (one bag for shirts, one for pants, etc).
Not sure if the link I posted was for the same size bag as mine. Mine is in the garage atm, but I can go get the dimensions if you want them. Overall, I am very happy with the purchase.