Snake,
Sharkshields are fairly common over here in Oz. I know a few people who have them. Oddly, a lot of folks buy them, do a few dives with them and then don't bother with them again. The units just sit in the garage or are advertised for sale.
One reported problem seems to be, unwanted shocks from the cable, especially when kneeling on the bottom or ferreting around in a cave. There also seems to be a bit of, "I never see any sharks anyway so why am I carrying this bit of extra kit ?".
The manufacturers site seems to contain some quite impressive footage of the units working as advertised, (But, then, I wouldn't expect to see anything else on a manufacturers website! ), but I have never heard of anybody actually using one 'in anger'.
Of course, they do say that the shark you need to fear is the one you can't see!