There was a murder there in 2006 and the guy was caught and got the death penalty. There were two robberies in 2008 by teen age groups. In one incident the victim was beaten pretty bad. There was another beating on the Wild Horse Trail, a Cobb County side trail in 2009. Nothing since then that I know of. None of the robberies involved weapons. They simply knocked down the rider or runner and overwhelmed them.
There is a real noticeable police presence on the trail these days, especially in Cobb and Paulding Counties. Police patrol on bicycle and ATV type carts and are friendly and helpful.
People recommend that you not travel alone on the trail, though I do it regularly. I am cautious around groups of people just standing around on the trail, especially teens, but I don't see this very often. If you are at all concerned about getting assaulted or robbed, DON'T TRAVEL ALONE.