http://injuryprevention.bmj.com/cgi/content/abstract/11/2/97
Quote from the above website:
Optimal position was defined as having the head restraint above the ears with the back of the head touching the head restraint.
Interesting reading, also from neurosurgeons.
The following came from Progressive Car Insurance and seems to dovetail with the above:
Next time you get in the car, follow these simple steps to help protect yourself from neck injuries:
1. Whether you're the driver or passenger, check out your head restraint before getting into the car. Get to know how it works. Does it adjust up and down? Does it tilt? Does it lock into place? Knowing how your head restraint works will help you find the best position for you.
2. Once you're in the car, be sure your seat back is in an upright and comfortable driving or sitting position.
3. Face forward and reach behind you to adjust the height of your head restraint. In the optimal position, the top of the head restraint should be as high as the top of your head and no lower than 2.5 inches below the top of your head. If your restraint locks into position, make sure it is locked in after you've found the right height for you.
4. Still facing forward, feel how close the head restraint is to your head. In the optimal position, it should be as close as possible to the back of your head, no farther than 2.5 inches away. If your restraint tilts, this can help you find the right distance. Adjusting the height of your seat can help, too.
5. To be sure you've positioned your head restraint properly, you or someone in the car with you can use a ruler to measure the height and distance from your head.
Dunno about you guys, but that tells me that I need to be using the top of the headrest (since there is no middle to ours) for measuring purposes.
And as an aside: they may be the best for crash considerations but they still don't have to be rock hard. SOME cushion would be nice since the framework of the restraint would be doing it's job in case of an accident.
Sorry this was so long but I thought it was useful to the discussion here.
Hope this helps a little.