knee probs will most likely be down to cleat and or saddle positioning.
I have tried time attac which were ok, also egg beaters which I didnt really like due to the lack of platform, also they are the only pedals that have caused me to fall over through not being able to unclip!
I now use shimano DX, mainly because i like the flat pedal feel and i find the extra support they give to be more comfortable.
they seem much easier to unclip from than regular shimano spd's, so i am having to wind down the release mechanism much further than i used to.