Basically, this study looked at college-aged distance runners and tracked the amount of injuries they incurred. They found that runners that have a mid-foot strike have a lower rate of injury than runners that have a heel strike.
So, what does this mean? When most people run, they have a heel strike. Their heel hits the ground first, then the foot rolls forward and they push off with their toes (mostly the big toe). With a mid-foot or forefoot strike, the ball of the foot comes down first, and there is very little roll of the foot. The whole foot is lifted off the ground. This article has very nice pictures illustrating the concept. Essentially, when hitting with a mid foot strike, there is much less impact to the foot, and, in turn, much less impact to the rest of the body.
I switched to a mid-foot strike a few years ago, following the plan outlined in the book ChiRunning by Danny Dreyer. I had be bothered by some nagging hip and foot pain, and it really did improve once I switched my stance. My speed, while never that fast, also improved.
So, the moral of the story is- if what you are doing with your running is working for you, don’t change anything. However, if you are having some foot, hip, knee, or back pain while running, you might want to look further into this. The ChiRunning site has some great information.