I have one, and love it. Not only is it a fraction of the weight of a bear bag, but it's volume also reduces as you eat your food.

I'm also pretty well convinced no-one ever actually hangs a bear bag properly, I mean, really, who among us hangs their food 200 feet from their tent, 12 feet up, six feet out and at least three from the top of the branch?

At this point, I'm pretty well convinced it's just as good as a hard sided canister, maybe even better. An Ursack gets tied off, so even if a bear mangles your food, you know where to find it. There's nothing to stop a bear from swatting your canister far enough away to render it unfindable.

Just 'cause it's fun, check out the video of the test