They are interesting to play with. Would never rely on one for my actual daily cooking needs. They do work quite well depending on the design and when/where you try to use them.
I've used something similar a lot more, a solarfood dryer. Makes great sun dried tomatoes.
Found plans a few years ago on-line.
We own Go Sun and love it! When our propane was low and we weren't sure when we would be able to refill, I cooked everything in it. Yes you need sun. Yes the food must be cut into long strips and is only good for 2 people. But we love it as a back up. Free energy!