I have seen one in person and felt it. Very nice, comes in many finishes. Very hard and actually non porus.
But, I have many questions as to cleaning, stains, and ESPECIALLY when the foundation settles and it has a mild bow or heave to it. We all know what concrete does on streets and sidewalks, why not on countertops?
As far as building one, I imagine it could be done, but I am not the one that would do it right off. I would have questions as; is there rebar or something running through the inside of the concrete? Are their some sort of expansion joints? Is this on some sort of floating supports that will flex to an extent. What do I do if the corner chips or cracks? Will it, the crack, run like a chipped windshield? Can I change the color? How difficult is it to change the sink or a cabinet out after a long period of time with that concrete counter top on top?