Use it all the time under my crusader for a quick brew, must admit to never having used it to cook over, i normally get the stuff going for a brew while i put my basha up (if it's not raining that is), by the time i have basha and hammock up, the brew is done.
In winter i keep two sachets on my person to keep it warm, but even the stuff in my pack seems to work ok for a brew, if i need to cook i use my trianga, or better still get a blaze going.
I dont think it serves the purpose for cooking over, but for a quick brew it's good enough.