But the one that brightredmtn linked to isn't heavy or sharp, and it attaches firmly. It STILL came off and caused the only injury her son sustained in a high speed rear end crash rearfacing.
Far be it for me to disagree with Jools (whose feet I would kiss if I ever met her in person). As a tech I completely agree with what she said.
As a mother, that's the one I have. I don't have adjustable headrests in the back, so I'm doing the best I can. What I tell parents at checks is that as a tech I cannot recommend any of them. As a mother I would tell them to weigh the risks. Their baby could get a bump on the head (or worse if they have anything that takes batteries. Those I actively recommend against). I tell them to think about throwing it at their baby. If their baby would be happy then it's fine in the car. If their baby would get a bump then it's something that needs to be weighed. Is it important enough to risk a bump to the head? If it could hurt them then it really needs to be weighed.
If they have adjustable headrests, and loop it around the headrest, then attach it to the tether, it would likely be a serious collision in which it came off and struck their baby. More likely the baby would rebound into it. However, I mention both possibilities. Generally, though, if you're in a collision that just ripped off a mirror that was securely attached around the headrest and then to the tether point, you've got more issues than a 10 ounce mirror being in the car.
For me, as a parent, a bump on the head would be worth the relief that I can see her when I need to and know what's going on. As a tech, I recommend against after market products and anything heavier than something I'd throw at her head (that goes for toys, sippy cups, shades on the windows, pets and their carriers, books, the diaper bag, a DVD player, etc.).
So weigh it, see how you feel, know you're going against recommendations, and make your decision, and you may have something there that could injure your child. Do NOT get anything that sings and dances. That will hurt your baby.
Wendy