I have Speedplay Light Actions, which are similar to the Zero but free float rather than adjustable. I came from the mid-range SPD-SL Shimano pedals and was quite happy with the Speedplays. They are no easier/harder to walk in than the Shimano cleats are, from my experience. It's a flat and fairly large platform on the shoe. I do find it easier to clip in in some ways (double sided pedals, only one place for them to go ) but harder in others (the SPD-SL pedals were easy to mate but harder to snap on in the back). Clipping out is definitely hands down to the Speedplays, I find it easier to get out when I want to with less torque on my hips and knees (and I have not accidentally clipped out to date while climbing or anything). I switched on the recommendation of my LBS when I set up my triathlon bike. My hips and knees are much happier (though YMMV, of course).

IIRC, the Shimano pedals are weighted so that they fall in such a way that if you point your toes toward the ground, you can "pick up" the pedal with your cleat, and it'll rotate back that way pretty well. Been a while, though.