guppy,
the EBS should not be confused with either a lifejacket or bouyancy aid, it is niether. It is a bladder that you inflate with your exhaled breath,it then scrubs and and recycles that breath for a limited period and does not impede your exit. It was designed for aircraft escape and is a great bit of kit. I have been trained over 20 years in underwater escape and when I first used one of these I noticed a significant difference in my attitude. Obviously the panic factor is removed (not for those with claustrophobia) and your first breath is air not water.
I have also used similar systems for escape from confined spaces (down inside oil platform legs) when they have been filled with gas and the only real problem I found was that the air become warmer and dried your throat. This lead to panic from some people but realistically when using this to exit an aircraft you will only use it for a matter of seconds and it will not be so bad.
Just to get you through the cold shock and long enough to get out the door.