There's a few products available like this already... http://www.hongkiat.com/blog/bluetooth-tracking-devices/
It's unobtrusive, and unless you actually leave them somewhere, they're unrecognizable from ordinary sunglasses. My only concern was being pestered about "forgetting" my sunglasses when I leave home into a rainy, overcast day, but they seem to have taken care of that too. The range display feature is just a cherry on top, I'd kill for something like that when I'm trying to find something.
I like this!
Using Vessyl as contrast, who gives a crap if my cup has a "unique yet subtle presence on the tabletop"? Does it keep my coffee hot? Great, then it's worth at least 5 dollars to me.
For sunglasses, design is paramount. It's a fashion statement first and foremost. Even without the "gimmick" the price wouldn't seem too outlandish, but at the same time you can immediately see how it would be useful in everyday use. And you do need a gimmick in a market controlled 70% by one single incumbent (Luxottica).
Come to think of it, the typical startup design is very much an Apple-centric cargo cult -- maybe how well it fits is somehow related to how Apple-like your startup is? (That's probably overthinking it a bit...)