Bike Helmet Mirror Review?!?

Does anyone actually need these?

I know you've seen these around, often worn by old men in spandex, most likely to arthritic to spin their neck around and look behind them. I'm joking, but I am curious about the necessity or even usefulness of something like this. It seems like another accessory for making money and, while it may be helpful in certain situations, is it worth having?

Bike Helmet Mirror

If I'm going to sacrifice style to where something like this attached to my helmet, I want to make sure that it is doing the job. Can I trust it to not have blind spots and send me right in front of a car? Probably not.