It's not enough for me; I wear a helmet each and every time I ride (well, except on recent bike-shares in Paris, Stockholm, Vienna, and Munich, where none were available). But I've crashed bicycles a number of times in my life, even once crashing into a car as a teenager (flipped, bounced off the car, hit the ground) and still never hit my head.

Anyway, it's just interesting to read about what causes head injuries and what makes a cyclist prone (or not) to them.