Is it a waist of money, or a good idea?
That depends on numerous factors. Firstly, are you kayak fishing where there are a lot of sharks? if not, then yes, waste of time. If yes, then it still depends...
Are you nervous about sharks, and do you know what you would do if you had a scary encounter? If yes nervous, and no idea what you'd do, then yes, a SS might be a good idea. Even if the thing doesn't actually work (they do) at least you'll be more confident and less likely to panic yourself into further trouble.
I had a great white encounter once (actually, had several, but one well publicized) and I did start using a SS for a while. But later realised that I wasn't turning it on even when sharks were around, so I got rid of it. I do kayak in sharky waters, but am confident I won't fall out, so therefore I have little to worry about. But everyone is different.
I dont like big sharks. Had what I think to be a great white follow me on a surfboard when I was a kid. It was big, like three times my length everyone said. Dillons beach in CA. Just not a fan of those. As far as panicking and falling out, I don't think so. I'm Marine, I worry for the toothy sucker that bites me. I would club it like a baby seal and tear through it like a dingo in a maturnity ward. But In whatever could go wrong, im a poster child, things bad happen to me allot. God hates me. I'm going to order one. I tournament bass fish and fresh fish for everything else in my hobbie or bass boat. Just getting into salt kayaking here, and don't have a clue of what I'm doing or what kinda fish I catch most of the time. I just took my first salt yak trip out of Van Damme south of fort brag in CA, rolled over twice and had to right myself in the surf, and kept going. But I will figure out. Looks like its a good investment in my butt since I swim about as much as I use the yak.
By the way, I'm still throwing out the I will take you on a guided bass/striper trip if someone can tell me how the hell I get through 6 to 8 foot breakers in a yak without flipping twice.