Since concussions usually come from the jarring of the brain against the inside of the skull due to a rapid change of direction like whiplash, the head gear is probably not going to do much to prevent a concussion in a big collision. That being said, I made my kid wear it for a couple years and still encourage her to do it (although she tends to "forget" to put it on). My mentality was that whatever cushioning it provided would be better than nothing, and it would certainly reduce cuts, abrasions, and contusions from head to head or elbow to head knocks. Even if it cut the force of the collision by 1%, I figured that's worth something.
No research on this point, but I also notice that field players who wear the bands tend to try to head the ball more with their forehead, where the band is, rather than the top of the head. In that way, it actually promotes better heading form and better control of the ball off the head. An unintended advantage, I think.
All anecdotal and unscientific, but I think most people who use them or bought them for their kids do recommend them.
