I have been researching these also. They have the small ones, but the bigger ones I"m interested in. My friend makes them out of propane tanks and uses sand to retain the heat. The idea is to burn the wood hot and fast, reflect enough heat to keep the coals and incomming air hot, and keep it hot for good secondary burn. You must size it for your application to get the best efficiency.
Because we use pine almost exclusively, the thermal mass stoves seem to be the best option for me. Optimum burn with lasting heat.
My plan for this winter, is to add some thermal mass to my woodstove in the form of concrete blocks. I'm also going to add some baffles. Let me know how it works out for you.
Are you thinking of building one for use inside the house? I haven't got that gutsy yet.