Are you wanting to use it for eventing or something else?
Several of my eventing friends own and use them and seem to like them just fine. For eventing you do have to wear a regular safety vest underneath it though. For regular riding you necessarily wouldn't, although if it malfunctioned or something happened like you and the horse both going down together, it wouldn't blow, so it wouldn't be very helpful at all in those types of situations. Or if it blows part way through a ride and you stop and take it off, then you are unprotected for the rest of the day/ride. Also I've seen them blow when a horse lets out a spook or buck and the rider gets unseated a bit or popped up out of the tack, and it actually *causes* a fall when the rider may have been able to ride out of the situation just fine without an air vest. The sound as it blows is a fairly substantial bang, and does spook or startle most horses.
Just my personal opinion, they are a neat idea and do provide a bit of extra protection for minor falls, but they don't really seem to me to be all that they're cracked up to be. I've been following them quite closely since they came on the eventing scene. The cartridges are a bit of a pain to change/refill, and can get pricey if you blow the vest a lot. If you are going to have a bad fall, or something happens and the horse lands on top of your, a little inflated vest isn't really going to help much at all. There isn't much concrete evidence or actual scientific studies done yet that convinces me they make enough of a difference to be worth the hefty price tag.