Have any of you used/had fitted a masonry stove?

Some friends have built a 'Mass Rocket Stove' (google it). It uses very little wood and the flue runs through a big bench made from rocks, clay etc. This makes a thermal mass that stays warm for days. They say that they come down to a lovely warm room in the mornings. It's all home made from low cost materials. If you are able to accommodate the bench into your designs then it's a very attractive prospect.

Similar principles, I think masonry stoves are used a lot in Germany as the pics I saw looked like stuff we've seen there.
My worry wine the rocket stove is the horizontal flue.

:-X