I bought this as a first time parent thinking it would come in handy and be very useful. It's still sat in the cupboard! I've used it once and it was ok, but I didn't achieve any better results than using a thermos flask or asking for a mug of hot water in a cafe/restaurant.
It would probably be more functional if in a remote area (e.g on the moors), but I find it much easier to prepare a fresh feed and let the water cool naturally in a thermal holder.
So not an essential purchase and just takes up space in the changing bag to be honest.