I have one of these in my 240Z. It allows me to see the blind spots better plus a much larger view than the stock rear view mirror. I bought mine in Japan but you can get it anywhere. They are spring loaded and go right on top of the stock mirror. You can still adjust the whole thing even, however it's a little PITA to finagle the sun visor.
As Jon said road racers love them, I guess it depends on your application. If it was for street use I wouldn't go with the wink mirror, track yes.