I had one on my Yama-haha... for about 5 minutes.
(1) They are terribly anoying to follow
(2) They don't put out any light whatsoever when the brakelight is 'on'
If you want a cutesy light on the back of your bike, there are plenty of others available instead of removing one of your major safety features.
If you aren't satisfied and want to try it anyway. Install it and then switch bikes with someone and follow them at night... then decide for yourself.