It is amazing to see how science and technology has evolve over the years.
With a sun oven like this one, bakers of bread and bakers of pasteries dont need to spend money on fuel and power to power up their oven, with a little perfection on this work this will revolutionalize the baking industries.
It works pretty good, I do not have a expensive one, but the main problem I would see is clouds can really effect the timing.