The last time I checked, there was no evidence that proper running is bad for one's knees.
Body parts like to be used.
Of course, you already know it's great for your cardiovascular, endocrine and muscular systems. Running improperly, however, can damage your knees, particularly if you have weak arches and your shoes are allowing them to collapse when your foot lands. Video feedback and coaching on running form is also invaluable. I think that it is hard on joints but that most exercise is. Running on a treadmill can be harder though because you have to take smaller steps and tend to come down hatrder on them.