This excellent product is by far the best eyeglass mirror I have owned. It is well designed, well made, very durable (I have lost some, but never worn any out) easy to use and adjust. It fits on either the right or left side of eyeglass frames, works nearly as well in drive-on-the-left countries as in drive-on-the-right countries.
In years of use, I have observed only two, minor, disadvantages: One is that they usually need a little adjustment when I put them on (easy), unlike some other mirrors that can be adjusted permanently. The other is that they do not sit well on very thin circular earpieces; they do work well on very thin flat earpieces. Like any other eyeglass mirror, their angle of view to the rear is limited.