We've been round this one on a number of previous threads, and I've posted this link before as well but for those who haven't read it, I thoroughly recommend Eddie Goggins' article in the ANR
http://www.aerobatics.org.uk/Journal_Nov_07.pdf#page=39
However, this is in the context of aerobatic types designed with emergency egress by parachute in mind - the average GA touring type was never designed on this basis, which in general makes the whole proposition unfeasible (and that's before we consider the dynamics of getting three or more passengers over the side).
Regarding SN3Guppy's comments - yes, as with any piece of emergency equipment, the user needs to understand the appropriate means of operation and practice correct maintenance and inspection procedures, but whilst I wear a parachute on most occasions I fly, I have yet to make a planned or unplanned descent using one - in the same way that I don't smack my head violently against the tailplane of the Yak whilst wearing a Gentex helmet to make sure I'll be familiar with the experience should I need to jump over the side, or spend a couple of hours bobbing about in a life raft in the North Atlantic to be sure I understand what exposure feels like - I fully expect that should I ever need to use an emergency parachute, it will be a violent, brutally painful experience that will almost certainly result in a couple of broken legs, but that the experience will be preferable to the alternative - frankly, you'd have to be mad to
want to use one.
