For about 6 months in 2012, I rode with both a bar-end mirror (Mirrycle) and an eyeglasses clip-on (Take-A-Look), because I was new to bicycle commuting and wanted to figure out which worked best for me, in the interest of safety.
It didn't take very long for me to find out that I strongly prefer the eyeglass clip-on mirror. I even was switching twice daily between clear safety glasses and sunglasses, and it was a quick, easy swap out.
The eyeglass clip-on mirror gave me the best quality image, and it truly became second nature after a couple of rides to just look up and left to see what is behind me. I sometimes now catch myself during the day looking up and to the left to see something behind me, it is that second-nature now.
The biggest drawbacks to me for the bar-end mirror were (1) skaky, lower-quality image, and (2) whenever I did any bike maintenance that involved flipping the bike over to place it upside down on the ground, the bar-end mirror got bent out of position.
I rode with both on the bike for at least 6 months, even though I really preferred the eyeglass clip-on and used it constantly. Toward the end of the bar-end mirror's tenure on the bike, I would occasionally look down at it when stopped for a red light and realize that it was out of alignment and probably had been all ride to that point, since I didn't use it. Still, I kept it on the bike because it did provide another view angle and it did provide a better-than-nothing insurance for the couple of times that I forgot to put glasses on for a short quick trip to the grocery store, etc. The bar end mirror eventually worked itself loose from too many times being bent when I flipped the bike over for maintenance, and it fell off my bike one morning in traffic at the corner of 5th and Laurel, to be crushed by the car behind me.
Mirrors are a personal choice, and I can see why a bar end would work well for some riders. I really prefer the eyeglass clip-on, though.
Here is an Amazon link:
http://www.amazon.com/Bike-Peddler-C.../dp/B001VTQNVO
and a picture:
