For mirrors in general, yes I find them extremely useful.
Take A Look Cycling Mirror Original Visor And Glasses - Cycle City - The Alameda Trek Bike Shop
I use this on my helmet visor. I used to put these types on my glasses but all these types of mirrors are a bit too heavy so they would slide my glasses down, but I also got headaches because the mirror was just too close to my left eye which is my farsighted eye. Swapped to my helmet visor and I get much better results and no headaches.
I see the car back well before I would hear it. Same goes with anyone on the bike paths catching up. I've had a few people break off a group and grab my wheel, I think the group was taking it a bit easy and these guys wanted to juice it up, and when I felt my pedaling lighten up, I was able to see the riders drafting. If I turn around to look, I will often go off line a little bit. The mirror makes it safer for me to check my rear. It is much safer to turn my head back and fourth a little bit to check behind me in traffic with the mirror when I commute to work since I go through downtown with the traffic around me.