Let's face it, running is a convenient form of exercise and can make you high. The only downside, I seem to experience is the occasional aching knee.
I take care of this important joint by strengthening my quads and rolling out my ITB. I do find it heartening that many studies have found that running doesn't contribute to osteoarthritis in the knee joint. Many of the studies on avid runners, weekend warriors and non-runners found that running did not increase the risk of developing arthritis. While little aches do creep up every now and then, the benefits of running seem to outweigh the pain.
Speaking of weight, it is true that every extra pound you carry on your frame creates an extra four pounds of stress on the knee. Running to stay in shape and keep off weight helps your knees, which in turn will keep you running. So get out there, and don't pound the pavement, run softly on the trail and enjoy your strong legs and healthy knees.