The Hit-Air looks interesting but I see a couple of problems with it:
First is the price ($499 list; $349 on E-Bay). The Casel Equi and Tipperary vests run about $250 (and you have lots of color choices).
Second is the issue of puncture protection. There's no mention on the E-Bay post about puncture resistance of the vest fabric. If you fall and the air bladder is punctured by whatever the rider falls on, all that air is going to be pretty non-protective.
In a fall or if dragged by a horse, I would much rather have something that's able to handle bumps, pokes, etc. I'm not sure I would trust an air bladder to provide that kind of protection.
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