I use two. One that extends from the end of the drop bars and a helmet mounted one. It is a momentary hassle to get everything in alignment, but then it is much safer.
I ride with guys who don't have one and I clearly have the advantage of seeing cars coming up the rear. This is especially useful when cars are approaching from the other direction. There's that momentary pucker when all of us are in alignment and I wonder if the driver is going to panic into my lane.
It beats just looking. The helmet one is kinda small so I got the one that extends out of my drops. Granted I am not cool looking, but I can see who is behind me and I can see them while still amintaining my view on the road in front of me.
The problem with the glasses mounted, which I had and loved was that dpending on manufacturer some don't fit well on some makes of sunglasses.
Either way, I won't ride without at least the helmet mirror now.