Operating SAR in northen Europe.
We are using HEED III, where the bottle needs to be lifted from it's pocket and placed in mouth. Other systems have a hose extension that I have no experience from, but might be better depending on how easy the bottle is to get out from it's stored position. There is no help with spare air if it is not easy to access!
Never had to use in anger, hope I never will, but in HUET training it is great to be able to take a couple of breaths to calm down, before exiting. My experience is that there is more air in that small bottle than advertised, but during hyperventilating it might be another situation. For a non-diver like myself it is really an experience to breathe below water.
If all theories are right the aircraft will not sink immidiately but there is a fair chanse of rolling upside down. Sitting still breathing (even upside down) until all violent motions stops, and then exit makes sence. Wonder if that theory will be remembered in a real case?
There was a military 412 accident in water 1 or 2 years ago where the inverted helicopter RH cockpit door was not possible to open. The pilot had to go out through the copilot door after first spending time with his own door (he made it though). That is an example where it would really have made a diffrence.
The HEED III have had good service record for us, some units have lost the pressure unexplained, but all in all they have worked very well. It is always the crews responcibilty to check their survuival gear though.