There is ABSOLUTELY NO PROOF that these devices attract sharks from further away. This is just an idea a few guys have dreamed up - I guess it comes from not wanting to be the only one in the water not wearing one? Personally I don't think they do, but the point is that we don't know, so to say they attract sharks is disingenuous at best.
I have one, and use it for shoredives upcoast. Who knows if they work or not, I have seen a huge shark while not wearing it, and I would rather have had it on. Having said that, I can't say that it makes me feel particularly secure, you wonder if it can really turn away a shark that big??
I have shot a number of fish wearing it, including shy cracker, so it is fine to shoot fish. It has chased away some spotted gully sharks, so I have seen it work on smaller sharks (up to 5 foot).
Cheers,
Tony