Corn has around 6,810 btu per # (if you consider the energy required to deal with 15% moisture content). By my figuring that is about half the value of coal which averages around 13,000 btu per #. According to the professor sited in the link many stove companies exaggerate the energy output of burning corn. Besides cleaning and drying are large variables in the process so just like with coal you can have widely varying results.
I have looked at several corn furnaces in operation, don't like the carmelized mess that seems to accumulate in the burn pot. As another Forum member said, its a crime to burn something that you can drink (or something to that effect
). Buyer beware, with all the corn being used to make alternative fuel for cars, it is bound to go up at a rapid rate.