There has to be a spectrum of susceptibility. We're discussing football, but the sport with the most repetitive head trauma is boxing. The worst damage inflicted is in the Heavyweight class (simple physics). Everyone mentions Ali and his serious mental/neurologic problems, but many championship heavyweights (Foreman, Frazier, the Klitschkos, Lennox Lewis, etc) seem to be absolutely fine. Lewis and the Klitschkos are even known as being pretty intelligent.
On the other hand, several times a year a boxer will end up brain-dead after taking a bad shot in a match. I enjoyed boxing and Muay Thai when I was much younger and think both sports are great for fitness, but I never had any desire to compete due to the concerns about taking one too many shots.