I ride Look Keos (with zero float) and have knee issues. Once I got a proper fitting including Aline Insoles (search here to find my thread on them) I have very little knee problems. My knees still swell some if I forget to take an anti-inflammatory but this is from years of abuse (bowling competitively, then running). Even in this case they will be bearable where as I used to cry at the pain when I rode SPD and then before the proper alignment fitting on the Looks.
I have the Keos on the least tension and disengage with the greatest of ease. No twisting, just a firm flick of the ankle. I do not clip out unexpectedly though. The single side is a little tricky but they tend to rest with the side the engage up. Usually one nudge and I am in, often no flicking around.
DH has rode both, switched from Speedplay to Look Keo and found the Speedplay easier to engage but I will caution he is a larger guy and a masher. I have heard smaller/lighter riders have issues with Speedplays. DH does find her is able to sprint better in the Keos.
Amanda
2011 Specialized Epic Comp 29er | Specialized Phenom | "Marie Laveau"
2007 Cannondale Synapse Carbon Road | Selle Italia Lady Gel Flow | "Miranda"
You don't have to be great to get started, but you do have to get started to be great. -Lee J. Colan