After spending thousands in my road bike, this the least expensive piece of equipment I have and probably one of the most effective!
* Easy to attach to either my sunglasses or clear/protective glasses, the part that attaches to the glasses is bendable so it will adjust without scratching the glasses.
* Mirror stays adjusted in place all the time during all my 25+ miles rides
* Mirror allows both sloping and horizontal angles adjustment, so you can have it in a position that won't interfere with your view
* Allows me to quickly verify the distance to any approaching vehicles or if it is clear to change lanes/making turns
* Portable, I can also use it when riding my MTB
* Inexpensive, although I think it should not cost more than $10 due to plastic contruction
Cons (None that I would consider as a product design or implementation issue):
* It takes a some time to get used and learn how to have it in the right position
* I need to turn my head a little to the left (15-25 degrees) when looking into the mirror in order to have it in the right position, but this would also be required if a had a fixed mirror attached to the handlebar.
Bottom line: Highly Recommended