I got one last summer because I ride where there is alot of traffic sometimes and I got tired of cars sneaking up on me. I got this one
http://www.amazon.com/Cycleaware-Ref...3410630&sr=8-1 , I took the double back tape off and put velcro on it instead. Now all I have to do is glance up at it to see what is behind me. No more cars sneaking up on me either. I tried a mirror on my handle bar first but there was to much vibration so I went with this mirror and I am very pleased.