I just got a pair of speedplays. What impressed me most about them was the freedom your knee has. It seems to me that w/ most clipless pedals your knee is locked in a position and only has a limited range of motion. When I tried the speedplays I had a much larger range of motion and it felt better on my knees. Also, when clipping in and out, you're not fighting against the friction of the pedal since the cleat is what clips on and you just push straight down onto the pedal.