Tried out a set of Power Grips on my KMX Typhoon, but they made my knees hurt. The angle of my foot on the pedal was too diagonal, I guess. Sold them and took a significant financial loss in the process. They probably work fine for other folks.
Same pain here. Additionally, I need to allow my feet to freely ankle-pivot to both left and right. That hugely serves to reduce arthritic pain, that HUGELY increases, when NO movement is available!