This mirror works fine, takes a couple times of riding to get used to it, but I found it adjusts easily and has plenty of range of motion for adjustment. If you adjust it correctly you simply turn your head a tiny bit to see cars behind you.
I would never rely on this mirror in checking traffic before making a turn, of course look over your shoulder for that! This is more to keep from a car coming up on you to surprise you or to warn you if someone is too close to the curb not giving you enough riding room.
(The instructions say that the mirror is unbreakable, which is nice to know that even if I don't survive the accident that I can pass the mirror down to my heirs : ))
There have been some other reviews that state the mounting arm is cheap and breaks easily, while this hasn't happened to me yet I have examined the mount and way it attaches to your glasses. There is a very easy solution if the mount breaks, it can be secured with inexpensive cable ties from your local hardware store. These cable ties are pennies, you can get a whole bag for a few dollars and they will easily secure the mirror to your glasses arm. The issue with this is you will need them semi-permanently mounted to your glasses which means you'll need a dedicated pair of glasses for bicycling. The ties can be carefully cut off with a razor or good knife if you need to undo the mount.
This unit is only $10 so you can't complain too much about the durability, the mirror itself is very durable and is optically of a very high quality, so I can't see complaining about this unit even if the mount breaks. Its only ten bucks. I give it 4.5 stars.
HINT: you may want to start off my leaving this mounted to a pair of glasses that you can use as a dedicated pair of riding glasses. This will probably prevent the mount from even breaking if you leave it on. Even if this isn't your first choice, it will make the unit last longer & it probably won't break at all. Taking the mount on and off every time you ride is going to put extra stress on the mount & of course it probably will eventually break. I leave mine mounted to a cheap pair of polarized glasses for $15 that rival my expensive Maui Jim glasses in performance.
HINT 2: Measure your glasses arm before you buy this. If the arms on your glasses are wider than about 1 cm you may have trouble getting this mount to sit right. If you wanted to mount it in a semi-permanent fashion with the cable ties I previously mentioned you should have no trouble on thicker arms, but I think this unit was designed to work with glasses arms that are not overly thick or not more than a cm wide.
Hope this has been helpful!