http://www.sospenders.com/product/8030206.html

Looks like a good idea, but how does it work in emergency?
They claim 22 to 35lb buoyancy.

Some have manual inflater if CO2 cartridge fails.
Some have pockets and even D-rings (makes it a harness too).

It does not make you hot and does not restrict movement.
It does not even cost more than good life jacket.

Where is the catch?