My parents have one. I can't comment specifically on savings etc other than my dad loves it because it saves a ton of money and my mom hates it because it takes way too long to get hot water.
The only other thing I know is that you can run out of hot water if you are using more than it produces. Not likely for most people unless you are running your dishwasher plus several people are showing at once or in quick succession. I would guess it depends on the model you buy as well.
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