Do the health benefits of long-distance running outweigh the costs?

Lots of folks I know have been taking up long distance (marathon) running - and usually cite it as a tremendous personal goal and an opportunity for a major endurance gain. Besides, there's the 'runner's high'. However, I have also heard from others, including folks in the medical profession, that training for and running in a marathon is a terrible thing to do - because no matter how much you attend to form, it is a lot of wear and tear to your joints that you will regret when you're older. Is there an official medical position on running marathons? Recommended or discouraged?