Are you talking the stuff made of beads? It does get messy if you cut or shape it. It's also not generally as structurally stout as the pink or blue extruded styrofoam, depending on what it's molded into. So if you can stand the messiness, it can be used if properly supported.
Mike Lehman
Urbana, IL
Is there anything wrong with using molded packing styrofoam for molding hills and mountain sub-structures?