I've tried several insoles, which I researched because of my knee problems. My knee problems have mostly gone away. I don't know how much of it was because of the insoles, exercises, or supplements. They probably all had something to do with it.
I think you have to assess whether your knee problems are due to your motion (ie. over-pronation) or the impact of hard courts. If it's due to over-pronation, you need support. If it's due to impact, you need cushioning. In my case, I think both had something to do with it, but my primary focus was on over-pronation because I knew I did it and I have weak feet (narrow, low arches). Asking for an insole with both support and cushioning is like asking for a racquet that has both power and control. At some point you have to give up one for the other.
The insoles I tried, in order of personal preference are:
Sof Sol FIT - great support, light weight, comfortable, little cushioning, favorite overall, comes in 3 arch sizes so it's almost like a custom insole. I currently wear the Neutral version for tennis.
Powerstep Pinnacle Max - great support, medium weight, a bit bulky, hard, a bit uncomfortable, no cushioning. It made my feet ache when I played tennis, so now I use them in my casual shoes.
Sof Sol Airr Orthotic - adequate support, great cushioning, heavy, best at shock absorption. I wear them in my gym shoes. If you play on hard courts and cushioning is more important than support, this is the one to get.
Sof Sol Stability - similar to Airr Orthotic but less cushioning.
New Balance 420 (aka. Lynco L420) - good support, some cushioning, has a heel post that causes your ankle to turn with the inner heel higher than the outer (supposed to minimize pronation). I felt kind of unbalanced on the court so I stopped using it.
Sorbothane Rx Sorbo Ultra - little support, little cushioning, hard, uncomfortable, and it was too straight to properly fit in any of my tennis shoes. Don't waste your time with this one.
