My wife wanted the Global Sun Solar Oven for Christmas this year so I got her one.$300 with the mess kit/baking pans,thermometer and a water pasteurization device.Here in Ohio where it's still only around 30 to 40 deg outside,I've gotten the oven up to 325 deg and have cooked a small kettle of beans along with a loaf pan of cornbread and they came out like you slow cooked them on the stove all day.The only downside (or upside depending on how you look at it) is it will not fry foods as it works like a convection oven,so it will bake bread but won't brown it,the upside is it also will not burn it.I've also found that if I don't seal the glass top lid,letting moisture and excess heat escape,it also works great as a food dehydrator.Also great for sterilizing water without fire/fuel.She loves it,so that's a win.win for me.
Those are the lenses that are used with the "direct heat" solar ovens. They only work in direct sunlight and must be adjusted to face the sun.
They can be used to make a solar forge, cast metals using the sun and a number of other things. As long as you have the sun and a warm day.