I wouldn't be without my Snuggle Safe 'hot water bottle.' Recently on a trip away I forgot to pack my Snuggle Safe (eeek!) but fortunately it was Amazon to the rescue, who delivered my replacement/spare to where I was staying the following day. Microwave it before bedtime, and it's still warm (not cold and clammy like a traditional hot water bottle) in the morning. Okay, so it's not particularly comfortable to lie on, but if it's chilly and you need warming up, pop it in bed with you and you're warm and toasty for the night. Having various health problems, including a long-standing back problem, I use mine all year round. Put it against your back for back pain or muscle spasms, on the tummy for period pains, put your feet on it to warm up cold toes, hold it against your ear for ear-ache...the list of uses is endless!
Fresh out of the microwave it's most definitely HOT! Eight hours (or more) later though and there's still a fair bit of warmth in it. It's perfect for anyone who struggles filling a normal hot water bottle (perfect for the elderly or disabled, as there's nothing to spill filling it). After a few minutes in the microwave (6 minutes in my 850 watt microwave - there's times shown in the instructions for various wattages) it really does stay warm ALL night, and unlike some microwaveable heat packs it's reuseable without having to go through any complicated rituals to get it into a state where it can be reused. The heat retaining gel stuff stays firmly inside its solid plastic cover (it looks kind of like a solid frisbee, just under an inch in depth) so there's no danger of leaks like you get with those flimsy gel-filled hot water bottles, and it comes complete with its own fleecy cover which is washable. It stays warm all night (did I mention that before?), which in my opinion is one of its best features, and as long as you have a microwave (though I'm sure I've read somewhere that you can warm it in hot water in a microwaveless emergency, though I'm not 100% on that one) it's so easy to use - no spilling boiling water trying to get it into the small opening of a hot water bottle, or fiddling to screw the cap on (after of course making sure you've not got loads of air in your hot water bottle).
Advantages: safe, stays warm all night, reusable, comes with its own fleecy cover
Disadvantages: a bit uncomfortable to lie on, requires a microwave to heat it
Conclusion: BUY ONE! You'll wonder why you didn't do it years ago!