The temperature is kept at -10°C without any problems. In summer, however, it is advisable to cool the box down first, as at ~30°C it takes about 20 minutes to reach the maximum minus temperature.
Basically, I would buy it again, or consider buying a second one. One for refrigeration, one for frozen food.

We bought the box last year in summer. It does what it promises, within a few minutes it is down to 5 degrees fridge temperature and for -10 degrees it needs maybe 20 minutes. When we go shopping for frozen food in the summer, we plug it into the electricity in the garage (because we only drive 10 minutes to the next bigger shop) so that it has the temperature before we leave.