If you're buying Speedplay's just for kick and don't care about the maintainance, fine. Like PCAD said, you get the Look cleats with more float.
If you're buying it to address knee problems, as I have contemplated on doing - DON'T! Fortunately I spoke to the right people before doing it and got the very responce that it was like "putting springs under a badly tuned engine", which I thought to be a good analagy. If you want the float to get around a knee problem, get a decent setup done and it will solve much more problems. The Speedplays will just hide the problem(s) in your setup and most likely it will only be a temporary solution.
Got my setup sorted - my knee issues disappeared, my overall comfort increased and my times decreased.
Just my 2c...