I never had the flaming stone, but I am interested and will grab one when I get a chance.
As for my personal tasting notes the austrian granit beer definately had a smoky, caramel flavor. Is it a novelty, maybe. I like brewing history, and I would guess alot of other brewers do as well. Traditional methods like decotion mashes are still practiced, archeologists scrape beer stone off of clay vessels, and cultivate yeast from prehistoric amber - so yeah I guess there is an audience for this sort of beer geekdom. The end result should be good beer, regardless of the method.