Having just come back from a trip where I earned some experience relevant to the ursack - bear can debate, I thought I'd share.
We spent two nights at the same location. We had 8 people and 2 dogs. We had 7 food storage devices between us. 4 bear cans. One Bearikade Weekender, one Garcia Machine Backpacker's cache, 2 bear vaults, one of the old green material ursacks and 2 of the newer white material ursacks (not sure if the material is different other than color but, as I describe below, it performed the same).
First night: 4 bear cans placed near each other. 2 white ursacks tied per product instructions to a tree near each other. Green ursack tied to a different tree per product instructions. Bear spends some time trying to open bear cans, playing bongos, grunting, sniffing, generally gets frustrated, and that's all we hear. Dogs barely even notice, but that's a whole separate discussion.
Next morning: Bear cans are fine. Barely moved. Ursacks are all severely mangled with tooth holes and lots of claw holes, but they are still tied to the tree and were not pulled open. One was pulled so tight by the bear that it took a half hour to untie it from the tree. Contents are completely mangled. Were talking dog food in my tooth brush and bear spit in my peanut butter and hot chocolate mix pouring out of the tooth holes. It's like the next day after a high school party when the parents are out of town. All three ursacks are like this. By the way, all the food in the Ursacks was in supposedly odor-proof OP sacks.
Here are some pics of my ursack post mangling:
Afternoon: 6 hour intense hail and rain storm with wrath of god lightning and thunder. Despite being under bushes, all of the bear cans take on some water, though most contents are saved because of contents being in individual sealed plastic bags. Some of the water may have entered when we tried to feed our families during the deluge. Not positive. One package of tortillas is a sticky gooey mess. Ursacks at this point are just holding trash (the night before's mess and some dog food).
Second night: We all sleep soundly following the storm. No one hears the bear.
Next morning: One ursack was pulled from the tree. The knot didn't hold. It was the only ursack that still had food. Dog food and top ramen. The two with trash were undisturbed. Mine was not the one that got pulled down so I don't know what knot my friend tied. The removed ursack has an approx 2 inch opening at the top where it should have been pulled all the way closed, so the botttom knot held (not the tree knot), but not tight enough the bear was still able to remove all the contents through this hole. The bear tactfully left the shredded plastic bags and a souvenir scat next to the empty ursack. (Yes, we reported the incident to the ranger.)
1) Ursacks do not protect your food from being useless to you. Even though the bear may not be able to open a properly stowed ursack, it will completely destroy it's contents. Thus, it may save the bear, but if you are relying on the ursack alone, your trip is over. Thus, maybe as Cameron suggests, it buys you time to fight for your food if you hear the bear, but that's it.
2) OP sacks are useless. I will no longer waste my money.
3) Ursacks will not shred, but they can be difficult to secure properly, and you don't know if you secured it properly until the bear tests it for you. If removed from the tree they are tied to, the bear will squeeze out the contents like toothpaste from a tube. Again, I don't know how well my friend secured his ursack and how well he closed the top opening. Mine stayed on the tree, but it is apparently critical to get the knot on the tree right and the one at the closure right too.
4) Bear cans work. Bears can't open them. I own a Bearikade Weekender and will be buying an Expedition before my next family trip. The ursack will stay at home, or just be used for toiletries and trash.
5) All of the bear cans can leak a bit if subjected to intense rain for many hours. Put everything in plastic bags. Maybe even the whole bear can in a plastic bag if weather is coming through. The BearVault, due to its complete cover lid, performed the best in the rain, and maybe only took on a bit of rain when opened. Not sure on that.
6) Bottom line: I want to love the ursack. I've used it for years. But it just can't do what I need it to do and I will pay the "penalty" in weight and cash to use a Bearikade whenever I travel in bear country. I still keep my base weight at around 10 to 11 pounds even with the Bearikade so for me it's worth it.
Hope this helps.