Like most reviewers here, I had over many years tried various types of mirrors for my racing bike and road bike all to no avail. I was really sceptical of all of them until I came across the Zefal Spin Bar Mirror. I read through most of the peoples' experiences and came to the conclusion that it was worth trying.
Simple to fit and first time out, a little fiddly to set up for my requirements but after a few miles, it became second nature to glance down at the mirror every 10 or 20 seconds if traffic behind me was busy but less often, out on the open road in the countryside. OK, some folk are critical of the fact that it is convex, so you make allowances for that and the plus side is that you get a wide view of the road behind.
You still make judgements according to what you tend to hear and back it up with visual rear view vision in the mirror. It is particularly useful for being aware of these larger freight trucks when you need to be fully alert if the road narrows or there is oncoming traffic as well. Put is simply, you become more confident in your whole approach to cycling on the road which is a very good thing. At night time, you can pick out vehicles early on from their headlights some way behind.
You can't put a price on safety but I would recommend this as a must for any cyclist who wants to enjoy riding around in either town or country with less fear or worries about what might be coming up behind you.
I will be ordering 2 more mirrors, one for my 42 year old eldest son who has taken up the sport enthusiastically and one for my other bike.