The Rhode Gear set-up is quite robust. I've had two -- one velcroed on to the end of the bars, the other one plugs into the bar end (making it practical for flat and drop bars). The plug-in one has a swivel at the base of the mirror. The bike's been dropped on the plug-in one several times, and it's been knocked by path chicanes, doorways, pillars and stuff. Still going strong, or would be if I still had it on a bike.

The Mirycle, on the other hand, was knocked against a fence around 15km/h and cracked the glass. Bad luck ever since...

When I use them, the mirrors can give me a sense of what is happening behind me, including the vehicle that might be hugging the gutter. It can help identify openings when I need to take a lane. How far a vehicle is away and its approach speed. And if you are riding with someone, it can help you moderate your pace if you are fortunate enough to be faster than they are. Oh yeah, and you can check the reaction on the face of that pedestrian that you've just cut-up on the shared pathway -- just like drivers do to you as a cyclist!

But it never replaces a glance over the shoulder before executing a manoeuvre in traffic.