I'm no expert, but from experience, running downhill is harder on the joints, always gets my knees, and as with everything there's a technique. although as ever the 'experts' differ in this. It does use different muscles so they need to train too.

just a thought 

Running downhill in a controlled fashion is a great way of generating leg speed (turnover/cadence) which you can then use on the flat.

You will develop stronger quads and pass loads of people in races as the vast majority of runners never use the effect of gravity to their advantage. Instead, they sit back and mash their quads through the braking effect.

Don't do that - go with the flow, your heart rate will drop and you will become a better runner