2nd what geoff said. I ride Keo black with a black cleat. The Speedplay always develop a "rocking" in the cleat as they wear but long before they wear-out. Causes me a ton of knee pain.
Great point on rocking (more than float).
A note on float is that Look float very freely, but a lot of folks miss the point that the float is different and not just in easy versus sticky float. A key point is that with Speedplay your whole foot pivots around the pedal body but with Look type systems the cleat is fixed at the point and floats at the back. That fixed point at the front isnt a huge difference, but is a slightly better brace point for the float versus the pivot with speedplay.
I've watched several fit sessions with Pro riders (and lots of others), and the wear and rocking are the most frequently addressed things that relate to the pedals themselves.
The float thing isn't as problematic as either:
1. (and this is number "1" buy miles) bad set up
2. Cleat wear (and while both systems cleats wear, the type of wear results in a lot bigger change in fit character for speedplay).
The thing with speedplay versus Shimano / Look / Campy is that the cleat wear with Speedplay creates way more instability due to the smaller platform.
The "do the math" Ad vert was one of the most misleading things I've ever seen. The suggested size of the contact patch isnt as important as where the contact takes place.
Take two camera Tri pods. One with 1" feet and the other with 1.5" feet. The answer to the question "which one is more stable?" sounds easy unless the 1" feet are spread out significantly farther apart...
One of the key components of cleat set up is getting the tilt of your foot correct. As speedplays wear out, they create multiples of tilt versus wide cleat pedal systems (and that makes for a lot more wobble)... As wide cleat type sustems wear, the wear a lot "flatter" and across a much larger area and dont create tilt and wobble nearly as bad as speedplay...
A key fix for a lot of pro's and people putting in a lot of miles is almost always down to cleat and pedal body replacement. For sponsored guys, that's not a problem. They can get bags of parts and make sure they're always tight and flat.
That's not as practical for most folks (including this web reviewer hack)...
Past that, I love using new Speedplay pedals (it's nothing to do with weight though, as the total weight difference for these on a three holed shoe versus Looks lightest is not any place near substantial). I can dial the float (and adjustment is exceptionally easy) to a reasonable spot and I love the entry and exit. But I don't use em. I don't like paying for the upkeep and I dont like having to constantly check things as they wear to make sure I don't wind up off the bike for a month from Knee issues related not to float, but the slop that develops. And even when brand new, I feel more stable in Look / Campy / Shimano systems...