Chris Swrote a review of Speedplay Zero Cromoly Pedals on January 24, 2016
Familiarity: I've put it through the wringer
The Speedplay X is a wonderful pedal, but the Zero is a step above. You get the same quick engagement of the cleat to the pedal, but you also can now control the float and the cleat is much more durable and less prone to breaking, wearing, and failing.
I appreciated the controlled float because my left knee has a tendency to flick out and my shoes frequently rubbed the crankarm when I had unrestricted float, but with an easy adjustment that stopped happening. Plus I got less wasted foot movement while sprinting or doing high-wattage efforts. I also haven't had a cleat failure yet, unlike my experience with the X pedals.
The only issue I've had with them are fairly minor. First, the cleat requires a bit of a break-in period. Once the cleat is worn, engaging the pedal is really easy. Unless fouled with dirt, where it becomes a different matter. Second, the float adjustment screws strip really easily. I can't do any more adjusting unless I buy new cleats.
But all in all these are small things. The pedals work and give me exactly the float that I need. If you race, the Zeros should get your first look.