Shark shields are excellent for repelling fellow divers!

I've had sharks around whilst spearing, bronzies, tigers and reefies.. they were only ever interested in the fish.
I've heard stories about the shields repelling bull sharks and other whalers but
if you get a monster fish turning up on your dive, your sharkshield won't stop you making a mess of your wetsuit..

Personally I wouldn't bother, but if it gives you peace of mind so you can enjoy your sports then it's probably worth the money.
ps - spearing is heaps more fun without scuba kit.. couldn't believe it when I moved over here and found it was legal!