While agreeing with what so many have already said, about how nothing will make a moron notice you, I do think it helps with the non-morons. I can say that since Shesaid and I are almost always riding together, I can notice a big difference in her visibility between when she's wearing Hi-Viz vs. not. A quick check in the mirror to make sure she's still back there is so much quicker when she is wearing the Hi-Viz. The while helmet vs. black is also quite noticeable. While it is true that I'm specifically looking for a motorcyclist in these instances, I feel it would be foolish to overlook just how much more visible said motorcyclist can be depending on gear.
But mostly, we just ride roads where there isn't anyone to see us.
She's much more visible here:
Than she is here:
She blends right in with the road surface, the guardrail, and the surrounding rocks is the grey.