I have wonky knees and ride Speedplay. If you are riding road, there are several models from which to choose. The Zero allows you to set how much float you have, and you can adjust inward and outward float separately.

I have the X2s, which do not have adjustable float. If I had a do-over, I'd get the Zeros. I can say that unclipping is absolutely effortless with the Speeplay, something that is not true of most pedals.

I was having some trouble with hotspots, so I decided to try Shimano's SPD-SL Ultegra (road) pedal. I did like the wider platform, although I didn't like that it was single sided. But what I really, really dislike was the amount of force needed to unclip. I felt like every time I unclipped (which could be a lot depending on the route I'm taking and time of day--I don't trackstand) I was doing damage to my knees.

Once I figured out that my hotspots were more a function of my socks (I only use very thin ones now), I sold the Shimanos on ebay and went back to my Speedplays.

During every pedals stroke I can tell my right knee goes a bit wonky and my heel moves outward. I'm sure I am losing a bit of power. But every time I see my foot move, I figure that is energy that is not being used to tear my knee apart, as it would with pedals with no or more limited float.