There were so many exultant reviews of this flask that I suspected the Thermos marketing department had been slaving over a hot keyboard. Even so, I ordered the Thermos TherMax Work Series 1.2 litre steel flask: It really does keep drinks hot for a long time, unlike the other two flasks I have had to chuck out.
To test it, I made a flaskful of tea without milk (I added cold milk to the mug when I poured it out, if you are interested). After 3 hours, the tea was as if newly made. After 12 hours, it was as if new tea had just been left cooling in the mug for a couple of minutes i.e. definitely perfectly drinkable and tasty. The difference was barely discernible from the 3-hour-old tea.
After 23 hours, it was just the acceptable side of lukewarm. It was not great but better than other flasks perform after about 4 or 5 hours -- and remember, I had opened the flask a couple of times by then and poured some tea out.
The flask is dear but worth it in my opinion. It will soon pay for itself in terms of motorway service station refreshments abandoned.
The flask must not be immersed in water during washing. It is made in China.