There was a good article on pink bike in 2009 here is a link http://www.pinkbike.com/news/Leatt-neck-brace-review-2009.html
As a few years have gone by since it would be interesting to see any follow up research,
I regularly use a leat brace as I feel that on balance it could make a difference, there is also a lot written in the motocross forums etc.
the only downside I feel is that when falling it’s hard to curl up and roll and absorb some of the inertia, feels more like a missile. having said that I have landed on my head and the brace did seem to transfer the force to my shoulders.
It would be interesting to hear some anecdotal evidence from the medical profession.