We got it yesterday, but didn't have time to try it out then (and it was a little overcast).
Tonight, we cooked bratwurst in it. Within 30 minutes it was 310F (the outside temp was 89F). This was at 4:30, so not at the sun's full strength.
One mistake we made is that we didn't use a covered pan. The moisture from the bratwurst condensed on the glass lens and blocked some of the sun's rays. The temp dropped to about 210F. They cooked for 1-2 hours (I'll admit it, I took a nap) reaching an internal temp of 150F. The final temp of the oven at 6:30 was 150F.
This was a great success. The brats were moist and well cooked. The next experiment will probably be cornbread. I'm not sure how to handle the covered pan issue, maybe cornbread won't be as big of a deal. Cast-iron isn't the most suitable for a solar-oven, but I'm thinking heating the pan to melt the butter won't use that much energy and once the pan is heated, it will work fine in the solar oven.