I used to own a pizza stone that seemed to transfer a lot of heat to my bread, even though it was relatively thin and didn't feel very dense. Bread and pizza would brown nicely on it, even though it was too small for baguettes. One day it cracked and I bought a different kind (Ceramic Chef brand). It's a little thicker and feels more porous, and I just can't get crisp bread on it even after pre-heating it in a 500F oven for 30 minutes. The new one is definitely a different type of stone. Maybe it's engineered for people who want to cook on rocks but don't like crust?
I'm excited about baking on a large slab of cast iron. I've had a 12" cast iron skillet for ~15 years and use it all the time.
[1] http://www.katom.com/067-1016905.html (store link; the manufacturer only describes it in detail in a PDF)