I saw one of these in person this past weekend. It seems built pretty well. I don't know what they put in the area between area above the inside and the outside, but when you knock on the surface, it's clearly got some sort of insulation inside, almost sounds solid.
Bad news is, I don't think there's hardly 14" between the front and the back of the middle stone... behind that there is a grate area that is used to dump ash into. ( could possibly be used to locate a pellet firepot, if someone wanted to try to make it wood pellet powered.
I would likely buy one if it was around $350, but at $500 it's a bit too much of a risk for me to take.. It is built better than I expected, but I just don't know if it's big enough for a good sized pizza... I also just have no idea if I'd really like it , or use it enough to justify the expense.