I decided to try Speedplay Zero pedals a few months ago, because I moved my cleats back a touch on my Specialized shoes and could not duplicate the position on my Sidis. Sidi apparently has not gotten the memo that we are all moving our cleats closer to the arch of our foot--your calves are not that strong, so why engage them so much? I also find that my left foot is finicky and inconsistent about the lateral angle it likes to pedal at. My heels are closer to the cranks, but my left foot is more so, and it seems sometimes like I am pushing against the limits of float on the Shimano Dura Ace pedals I normally use. Finally, I am retarded when trying to get into my pedals at the start of a race. I can be relied upon to fuss around for a few seconds trying to get in while everyone else takes off.
Oh, and Spartacus uses Speedplays.....'nuff said.
Speedplays are renowned for their adjustability, and having a 2-sided pedal makes it easier to get into. They are a bit lighter and allegedly more aero, but I really did not care about that.
So I bought some. Turns out that I still could not get the cleats mounted far enough back. No problem--some hunting and $40 and I found the extended base plate that gives a ton of fore-aft adjustment. Oh, and another $40 for some angled wedges. That makes the total investment a bit pricey. I purchased the Stainless axle version of the Speedplay Zeros, as they are much cheaper than the Ti, and stiffer and more durable.
I have been using the Speedplays on my carbon bike, which I have ridden most of the time recently, but I am still putting in days on my Ti bike with the Shimano pedals.
I definitely like the new cleat position....it just feels very natural. Remember that you have to bring your saddle height down if you move your cleats back.
On the bike, I like the Speedplays a lot. Engagement is a bit easier, though not foolproof. I still manage to miss sometimes, and when you do, the consequences are more catastrophic that with the Shimanos because your shoe flies off the pedal. The feel is a bit odd, because there is no resistance to the float. At least with the Zeros you can set the limits of the float, but there is a bit of an ice rink feel, though I quickly adjusted to that.
The Speedplay design is different in that it is the reverse of most pedals--the retention mechanism is on the shoe, not the pedal. Effectively the pedal is the cleat.
The cleats are a pain. I have already had them come loose twice and slide laterally. If you tighten them too much, then they don't work properly because you create too much friction for the ring that is the retention mechanism. I tightened them to the torque spec but even with the threadlock on the screws they don't appear to want to stay in place. So, I finally just overtightened them and accepted slightly balkier function.
You need to keep debris out of the cleats, and the cleats are metal, so they are slippery to walk on. That means you really need cleat covers, which are a pain.
The Shimano cleats, by comparison, are just easy to deal with, and you can walk for miles on them.
For the riding I do, which involves a lot of walking around in coffee shops, I think the Shimano pedals work better. If racing performance is a priority or you have fit issues, the Speedplays get the nod.