Mainly weight. I couldn't tell you how the stiffness varies over the normal plastic one. People say that Sidis are a little more flexible in the sole than other shoes. Personally I can probably agree with that.

Some people believe that a more flexible sole leads to hot spots. All I can say is that I have had hot spots with other shoes, but never with my Sidis.

I can bear Shimano shoes, but little else (have not tried Specialized).