I have clicky creaky knees that are especially troublesome when going down hill, down the stairs etc. Most of the time my knees are not painful, but when they do get painful I simply strap a pair of these on and the pain is cured.
I have also bought my dad 2 of these knees straps, he uses them for playing golf. He had found it impossible to get around the course without pain, now he can play all 18 holes with no trouble at all.
I believe they work by putting pressure on the patella under the knee which then holds the kneecap into its groove on the knee, making it track up and down correctly, and eliminating sideways movement that causes the pain.