Sounds like a solution in search of a problem. I am not a biker, but I can't see how a helmet-attached mirror could be large enough or stable enough to be of benefit. It could even be more dangerous by leading you to think that it was efficient and yet having a larger or different blind spot than your handlebar mirror or a quick over-the-shoulder peek.
Have one on the left handle bar. But it never stays in place and is always at a bad angle but I do try to adjust it and do check it but its not a replacement for listening for cars and turning my head.
I turn my head. Riding in densely populated NJ can be somewhat nerve wracking. I turn my head as often as I feel I need to in order to know if someone is on my tail. That can be several quick turns within a 10 second span as a line of cars is passing me. I wouldn't think of trying a mirror. Just seems like overkill for not much benefit.
I use one on the bar end. It works well and stays put. However it's no replacement for looking over your shoulder. That should be your primary. But it's nice to have that mirror for glancing towards the back, especially in a pace line, etc.