We installed a Bosch Aquastar about 8 years ago.
It cost a lot to put in, because it required a bigger supply line and a bigger, double wall vent.
I doubt it really saves much gas vs. the tank heater it replaced.
With the tank heater, the pilot light keeps the water hot. when I shower, about the time I finish, the 30,000 BTU burner starts up, and runs 10 or 15 minutes. With the tankless, there's no continuously burning pilot, but the 175,000 BTU burner runs the whole time.
Also, I found that when doing dishes I needed to let the hot water run continuously in order to have hot water at the sink. Every time you turn the water off, the burner goes out. When you turn the water on, then cold water enters the hot water pipe until there is enough flow for the pilot to light and the main burner to kick in. The hot water mixes with the cold in the pipe, and I ended up with tepid water at the faucet.