I think they're great and when/if I get to prelim or higher I will definitely get one. But I would never wear it without a certified vest underneath. It won't protect you at all if both horse and rider fall together (rotational fall) since the chord wont snap unless you happen to pull it yourself but I doubt youd be thinking about that at the time. Haha.
I think they are a great idea, and I am with Drew - that vest saved Olivers hiney at Rolex, but they are very expensive to replace.
The air canisters that cause the vest to inflate, are around $60+ to replace..yeesh...to much for my purse. I'll stick with my vest for now, until I get to higher levels and invest in a Rhodney Powell.
It's the canisters that are expensive to continue to replace. The vest isn't "re-usable" as easily as a regular vest is. But that's because regular vests do not rely on the operation of the air canister to do its job.