I have found a helmet-attached rearview mirror is more useful than one that is attached to the bike.

It is not a substitute for a head turn but it is a good indication of what is coming up from behind for a long way off.

The only drawback I have found to the mirror is that the manufacturer issues it with a pathetic starburst-shaped sticky pad that works for five seconds. I replaced it with foam-based two-sided tape and have been very pleased with its durability.

I see relatively few people here riding with helmets. I use mine religiously since i would rather look stupid than dead.