I was used to Time ATAC's on my previous bikes, and really struggled in the beginning when I switched to Speedplay Zero's on my road bikes.

1) It took me time to change how I engaged the pedals ... I was used to pushing forward and down at an angle and the Speedplays need you to step straight down
2) At <115 pounds, I think I lacked the force required for Zero's and would have been better off with the Light Action. I spent a lot of time on the trainer getting in and out to loosen the spring on the cleat, which I also lube'd frequently.
3) Shoe covers are a must, as others reported ... these cleats will gather dirt and more as you walk around which will then prevent you from engaging
4) On positive, I have never experienced difficulties disengaging so no safety / falls coming to a stop

After a frustrating break in period I have eventually become a fan and have Speedplay Zero's on all my road bikes (and continue with the Time ATAC's on my touring/cyclocross bikes).