I am on the hunt for a pizza stone. I have become slightly obsessed with homemade pizza, and National Pizza Month is a good excuse to further the search.
In general, if you want to make pizza at home, a pizza stone is a good thing. The goal when baking a proper thin-crust pizza is to raise the heat and lower the baking time as much as possible. A stone absorbs heat as the oven heats up and ideally it will absorb enough to actually be an extra source of heat once the pizza goes into the oven - effectively raising the temperature even higher.
The reason I am still hunting, however, is because I have read some troubling things about the $12-$15 stones you find at home stores.
Often they shatter at the high heats necessary for pizza, and I have already had one do this to me. One eGullet member recommended the expensive FibraMent, a stone formulated specifically for baking. Many other people just use quarry tiles available at the hardware store. The problem with these is that they are small and often too thin. So I am still searching and weighing the options.