After riding clips for years, I've been clipless for the last 3 years--I'd never go back. Clipless allows me to more effectively push through the top and (especially)pull through the bottom of the pedal stroke, which gets me a few extra RPM when I remember to do it (still trying to get to the point where it's automatic). It's really a matter of being able to apply power through as many degrees of the pedal stroke as possible--for me clipless makes it way easier.