That's basically the only way I can afford to heat water.
Installed ours in 1999; only maintenance has been an occasional puff of compressed air around the pilot flame (while turned off) to blow away cobwebs.
I did have to insulate better around the unit as it froze up during the aforementioned -17F. They are more susceptible to freezing since the water is only warmed while the unit is operating, of course.
Yeah, watch out for bugs in heaters.
Few years ago, my furnace wouldn’t start up in the fall, and I eventually found the problem...
Some damn bug got in the main gas pipe going to the burner, and built a nest there. It wasn’t completely plugged, but it created enough back pressure that the gas valve refused to stay open (safety feature).