The Yak Sak is as a tough as any roll-top bag, and is waterproof when closed adequately. You need to roll the top at least twice to seal it well, and each additional roll diminishes the volume of the bag significantly. Bob claims a Yak Sak volume of 5600 cubic inches/92 liters (they evidently can't do unit conversions accurately), but every way I've ever measured it I get ~4500 ci with the top rolled only once (not a secure closure), and close to 4000ci with a proper double roll closure.

The Yak Sak weighs about 4.5 lbs (I've weighed it). BOB has never mentioned this in their specs all these years. They DO say the trailer weighs 13.5#, which is true, but when you add the bag (which most people use) it totals 18 lbs. This is a little trick they do to make their trailer sound comparable to their main competitor, the Burley Nomad. The Burley Nomad claims 105 liters space under the hood, with total weight of 14.8 lbs - it's lighter and holds more stuff. You can stack stuff on top of the Yak Sak for more capacity, but you can do the same with the Nomad if you add the accessory rack. Bob carries 70 lbs, Nomad carries 100 lbs (according to specs). Also worth noting Nomad isn't 100% waterproof under hood - you need to store electronics in another WP bag, but you'd probably do this also with the BOB Yak.

As an alternative to the Yak Sak, Ortlieb makes a bag that's larger, weighs less and and fits the BOB Yak very well - available in several colors, even the same yellow BOB Yak color. The Ortlieb Rack Pack XL weighs nearly 2 lbs less than the Yak Sak but costs $20 more, which may be a fair deal to many people. Plus it could color match your Ortlieb panniers. It's been discussed here before.

http://www.ortliebusa.com/CartGenie/prod-62.htm

http://www.ortlieb.com/_pdf_en/rackpack.pdf

http://www.bikebagshop.com/ortlieb-r...xl-p-1039.html

http://www.burley.com/home/bur/page_454/nomad.html

http://www.bobgear.com/yak

http://www.bobgear.com/trailers/drysak