For those of you that haven't experienced it, I have brought my zojirushi filled with hot (freshly boiled) water on a 2-day ski trip where I left the bottle in the car during below-freezing temps. At the end of the first day (approximately...18 hrs?) it was still too hot to directly drink (but felt pretty good after the cold). At the end of the second day, maybe..30-ish hrs? It was still a bit warmer than "lukewarm". That's pretty impressive considering the external temps.
At room temp I believe it has kept my liquids hot for over 48 hrs.
Of course this is all anecdotal, but if you need to maintain temperature for a long time, the Zojirushi blows EVERYTHING else away.