: I don't have one and don't know a whole lot about them except that they are rarely installed by a builder. I suspect that there is a good reason for that.

Mostly that they are unfamiliar with them and they are more expensive up front.

: The controls are complex and the burner or element is very large compared to standard water heaters. I would expect problems and maintenance costs to be proportionatly larger

The controls aren't all that complex.
All parts on the three AquaStars I have installed are replaceable.
The three have had no service in the past six years.

There are very real environmental and financial savings over the long run.
Plus folks can shower one right after the other for as long as you have gas and water.

Good health, Weogo