if you're going to get a shark shield, the model you will probably want is the "freedom 4". it's the most compact and has about 4 hr battery life.
some people say they attract sharks, i would say more people who wear them get away with seeing sharks.
i never heard of anyone getting eaten while wearing one though.
you can sometimes pick them up for $400-$500.
i think they are manufactured in Adelaide where they get a few big sharks.
I think you also need to be wearing a wettie (boardies in or out shouldn't matter

.) Other wise you get a weeny bit of a shock from it.
oh, and i don't wear one, looked into it a bit but haven't had anything scary enough yet.