At work, I have to walk up and down a flight of stairs, usually about 2 - 3 times/day.
Usually, my knees ache a little when going down the stairs slowly.
I just started to notice that my knees don't hurt, or hurt less then normal the day after I do squats. I wanted to plug squats.
When done with the correct form, they can strengthen your knees.
Ironically, leg extensions, the lift most people do who are afraid to hurt their knees, cause me knees to ache more than usual.