The water heater that came with the house leaked. A lot.
We want to be eco-friendly where possible, so Rob did some research and chose the AO Smith GDHE-50 to replace it. Down the road, we plant to use it for hydronic radiant floor so that we can get rid of the monstrosity in our living room.
The AO Smith is supposed to be even more efficient than a tankless water heater. By a fluke, our first gas bill covered up to the day before we installed it, so in a few weeks we will be able to do a pretty accurate comparison.
It was pretty expensive but, thankfully, it's eligible for the federal tax credit.