Ok - let's first address the elephant in the room - they're ludicrously expensive.
That being what it is, what are people's thoughts on the DESIGN of them? I played around with a couple of of them in the store and I'm still not sold on the zipper concept. How long it will last and if everyone will remember to fully zip it (the only way it's fully watertight).
I briefly played with two different sizes and filled them with air and sat on them. They didn't budge - even when bouncing.
I'm just not sure of the longevity of them and if the zipper could get fouled enough to let water in during a submerged situation.