After much research (reading forums, talking to the LBS, asking those who ride them, briefly trying them) I bought the Speedplay Zeros last year as a transition from my SPD pedals.

I had read all about them taking care of knees and being good on cornering and great for entry/exit.

The verdict for me: they were excellent to clip in/out (double-sided entry is awesome) and were excellent on corners...on the knees, not so much.

What I found was that there were too many moving parts involved with the cleats for my body - adapters, shims, plates, springs - and that the cleat never really felt like part of the pedal, which I had read so much about. I was hoping this would be the case, but the platform never felt like more than the lollipop center.

That being the case, I could never quite get the pedal stroke down to avoid irritating my knees in various locations - side, front, back, inside, you name it (yes, my bike fits). I felt as if my foot could never find the middle and was always "rocking" from side to side. Pushing up hills and in large gears always exacerbated the issues.

So I switched to the Ultegra 6700 today, which matches my other components. On the ride to the office this morning, my feet felt much more stable and I was encouraged with the initial experience. While I'm obviously hoping that the Ultegras take care of the "rocking" and thus eliminate any pain I have, I'll miss the double-sided entry on the Speedplays.

I know my experience (just like any on these forums) only applies to me, but I wanted to contribute to the board and give this perspective to anyone considering a pedal switch.