These compact and affordable mirrors have an unobtrusive, minimalist design that slide onto your eyeglass frame. They're not nearly as clunky as some other mirrors.
I'm a long time user of mirrors. The small field of view takes some getting used to compared to the larger mirrors out there, but Messenger Mirrors are indeed usable.
I recommend mirrors for those who like to be helpful during group rides by yelling "Car back!" every time a vehicle approaches. Kidding.
I like mirrors because they enable me to check back quickly for merges, passing, left turns, and the occasional quick escape. I still do a shoulder check, but the mirror let's me know when a possible gap opens up for my move.
The only real drawback: The thin wire stalk doesn't lend itself well to easy adjustment -- once you have the mirror where you like it, it's a very good idea to keep it there.
Otherwise, the mirror works as advertised. I don't find the small field of view a hindrance and it works adequately for me, and the price -- $4.99 + 88 cents postage -- can't be beat.