I have not used this mirror. But when I started looking, my son (he and his wife are car-free, full time bike commuters), had this mirror on one of his bikes. He said the mirror has limited range and it's not easy to scan for traffic. He's had better visibility with helmet or eye glass mounted mirrors.

But why not have both. I could see having both a bar end mirror and an eye glass mounted mirror.

My son bought me a Take-a-Look mirror for my birthday. After using this mirror for almost a year now, I would never ride without one. As someone else mentioned, it feels naked not using a mirror.