When I was a kid I used to cook hotdogs using a suntwist that my brother in law made. I think he built it as a boy scout project. It was basically a 4' diameter reflector cut into spirals that reflected the light to a focal point. Seem's like it took about 5 minutes to cook some dogs, although it was better if you flipped them over and then it took about 8 minutes. That was the extent of using it for cooking except for marshallows.
I've often though of buying a solar oven but for the cost, I could buy a heck of a lot more propane. I think their usefulness depends alot on your climate and how much sun you really get. May work great in Arizona, less so in Washington. I consider them more of a novelty, nothing wrong with them but I haven't spent my money on one yet.
A man who has a stomach half full has lot's of problems. A man who's stomach is empty only has one.