Any cop will tell you that in crowd situations you don't have to make your stuff thief-proof, you just have to make your stuff harder to steal than the next person's. For that, this works, but so does the chained slash-proof bag.
This product is supposedly designed to protect valuables from being snatched while you sleep by having you lie on top of them. I have to ask what difference there is between this and a double-ziploc in that case; as if you are sleeping you are not going to be wanting to access them.
It would be a much better idea to pitch it as a place to leave valuables while you are in the water, but for that to work you would pretty much have to get there before anyone else, so that they don't see what valuables you are stashing. It does have an advantage over a chained bag because it would take effort to find, but let me tell you, unless it has a beacon in it, it would also be darned easy to lose. DD once buried one of her shoes when making a sand castle, and we were totally unable to find it, even though we knew where it had to be within a radius of 5 feet. Sand shifts really quickly all on its own unless you are camped below the tide line, and what kind of goof would do that?
We use an anchor sack with a hole in the bottom in addition to the anchor to help keep our umbrella up, and we make it a habit to put valuables in a double-baggie and put them it down at the bottom of the sack next to the pole. If we have to get to the phone or car keys for an emergency, pulling out the umbrella will enable you to get to the baggie very fast, but you have to essentially dis-assemble the camp areas to do it. No thief is going to go to all that trouble unless he knows there is going to be a worthwhile payoff at the end -- it's much less trouble to just grab that iPhone out of an unguarded person's hand and run for it through the crowd.
