zojirushi and elia are two brands i can recommend.
i've found flasks to be much pot luck.
cheap ones are cheap for a reason.
expensive ones are more likely to guarantee they'll work to expectation - more or less if they don't self destruct.
"light" flasks are light probably because they won't keep anything hot. a lifeventure one i had was light and kept stuff hot for a good two hours. the zoji is heavier and keeps stuff hot for at least 17 hours (gave up measuring the temp after that) and realistically when using the contents throughout the day a good 8 hours plus.
since acquiring mine i haven't looked back and was money very well spent. the linked thread below is now 5 years old! is it really that long i've had my flasks!
flask
Edited: 18/04/2012 at 13:50