Have you surfed the Speedplay web site? It tells you what they do that's so different than the other pedal systems.

Their innovation is about the pedal and its platform. In most pedal systems, the pedal is big because its the platform. In Speedplay, they do the opposite. The pedal is small and the cleat ends up being the platform. Speedplay thinks this is better for a number of reasons.

The platform is supposed to be rigid and so the Speedplay cleats are big with a lot of spring tension on them to connect to the small pedals. The cleat is connected to the shoe by those famous 4 little screws.

Those 4 screws eventually get loose in time, so you have to check them periodically otherwise you'll be out on a ride with cleats half off. Some say to use a lock-tight product to keep those screws in place. Speedplay admits the 4 screws are adjustable but doesn't mention about using a lock-tight to keep them from falling off. Maybe its a product liability thing. If you inspect the cleat screws before each and every ride, you shouldn't have problems. Then every 3,000 miles or so, replace the cleats (ie spend more money).

Two advantages: float and ground clearance (and maybe weight).

But if you check their web site, they have a good FAQ: question and answer thing.