i love brathplay and it's extremely rare we dont use it, the best part is when you're goin g to cum it makes such a difference and really heightens the sensations it can also make me cum on its own as well.
as you said it is very dangerous and although any fatalities are very low it has the highest mortality % of any bdsm activities.
people often over look the fact that problems can occur hours after playing, the lack of supply of oxygen to the brain doesnt always take affect immediately.
more than with anything else keep a very close eye on the sub for some time afterwards and ANY sign of something being not quite right, get it checked out.

this article by jay wiseman is well worth checking out.
http://epedominion.net/HealthSafty/breath.htm