I love these retainers!
If you're looking for a functional way to retain your glasses that doesn't involve placing them on top of your head, this is the way to go. I was concerned that the ends of the retainer would be too small for the arms of my glasses, but that wasn't the case; they fit perfectly. (I have something similar to the "MEC Ashton Sunglasses").
A word of caution to those who would want to frequently remove and re-install the retainer; They fit snuggly and it will be inconvinent to take them off and put them back on. Having said that, it can be done, it just won't be convenient. Once they're on, they are best to be kept on.