I have a similar unit (think its a waeco, havn't used it for about 10 years) which I used to use to keep my drinks/sandwichs cool. I was doing 16+ hr days at the time, so stopping for food was a pain. I found for it to be any use, it had to be powered perminantly. When powered all the time, it was pretty good. If you get it cold, it will stay cool (maybe not cold) even in very hot conditions. If you let it heat up, it will struggle all day. I would never for instance carry milk in one.
These days I use a fridge, and would find it hard to go back to using it. Having said that, I did get my value for money out of it.
I only used it on heat a few times in winter. If I was travelling in a remote area I would put it on heat a couple of hours before hitting a town. Buy a pie or sausage roll. Throw it in, and eat it later in the day when I was in area where I wouldnt buy food (some towns you just dont buy food in) It was enough to keep it eatable, but you would never burn the roof of your mouth.
Was a long time ago, and I have a cwap memory.