I got in on group buy and have one of these about a year ago, the "sport" solar oven I believe: http://www.solarovens.org/
. Admittedly I haven't used it near enough to master it but it will damn sure cook up some food.
So far I've cooked chicken and turkey (chili) and all kinds of non meats which are much easier. It really doesn't seem to need a large amount of sun so the assertion that this would probably only work in notoriously hot/sunny areas I believe is incorrect. Depending on your budget, I think these things are ridiculously cheap since the fuel is so plentiful and free and all preppers should have one.
I know a lot of folks store high quantities of rice and beans and I wonder how many have calculated just how much fuel it would take to cook up say 100lbs of beans over a few months. Re-run those calculations with a solar oven and you'll find they're invaluable.