If you fill one with icy slush it softens pretty quickly, but you're right that it's a handy thing.@Toddy you got me laughing about the hot water bottle front...
Reminded me of a fair few years ago I was at my (now wife) girlfriends house (her parents house) and her folks went off to bed...an hour later we went up and heard a hairdryer going off in their room and their light was on - a little odd for so late but nevermind, we started watching a movie in her room and the hair drying seemed to go on for another 15 mins, by which point GF decided to go knock on their door to see what was going on... apparently her mum was sitting in bed upright trying to dry the bed and her PJ's after one of her many ancient rubber hot water bottles unexpectedly gave up the ghost...
I'm not a very well behaved son in law, so naturally the jokes came out the next day about wetting the bed and adult nappies etc..
Actually, ever tried a cold water bottle? Quite handy for summer heat!
It sort of ends up wet with condensation though, but if you wrap it in a towel that towel becomes a cold damp thing Ideal on a hot day or muggy night.