When I got them, there was only the X-series (X-1 and X-2, with X-3 and the Light Action added later). Speedplay eventually released the "Zeros", short for "X-0", although really a separate design. According to someone I recently chatted with, Speedplay Zeros are considered by Speedplay to be "mechanically superior in every way" to the "legacy" X-series.
Well, "superior" except that they were more expensive and are heavier: 8 grams more for the cleats. Okay, 8 grams isn't much, but 8 here, 8 there, and eventually you're talking real mass, so I take it where I can get it. At my $3.50/gram threshold, 8 grams is like a $28 price increase on the pedal (people pay a lot more than $3.50 to save a gram when they get carbon bars, super-light brand-name bottle cages, or top end frames like the Trek Red-series). So there had better be an offsetting advantage to the added weight. And a quick check of list prices shows they're not more expensive any more.
The big selling point to the Zeros is the limit adjust. Speedplays were revolutionary among cleats in the amount of rotational float they offered (at the expense of no side-to-side float, and so you need to make sure the cleats are well adjusted). Some people don't like all that rotational float, for example sloppy sprinters who might accidently unclip, and so the Zeros address their concern. I love the rotational float, so have had no motivation to switch.
But the biggest weakness, by far, of Speedplays is the cleats. They wear easily, and when the X-series cleats wear, the retention spring can come into contact with the pedal spindle, and without much warning you end up with something like this (photo stolen from BikeTechReview forum):
The guy who posted this photo thought it was a warranty issue, an assertion I find completely absurd. It's a design weakness, sure, but the damage is strictly related to user error and to wear. But a common one. I've had this problem, I spoke to someone else today who had this problem, and obviously the guy posting the photo had the problem.
Today, having taken a close look at the Zero cleat, I realized the Zero may well be relatively immune from this problem... another stolen image, this one from Bike Sport Michigan:
That nasty spring on the side of the X-series cleat, the spring which contacts the spindle, is missing in Zero.
I really like my X-1's. But since Metrigear is only supporting the Zeros with their Vector force/power meter, I may find myself using Zeros soon. If they solve this wear issue, the switch may be worthwhile.
It's a good thing the Zero doesn't suffer this problem from the Vector perspective. Wearing a groove into the spindle, independent of safety concerns, is going to change the mechanical properties of the spindle, changing the relationship between bending moment and deflection, and cause an error in the Vector force determination. The solution would be recalibration (send the pedals back to Metrigear). But the point may be moot if they don't support X, even though the spindles are equivalent.