I bought a Takagi unit online a few years back to replace a worn out tanked unit. I had found a lot of information on the unit before I purchased. I was primarily interested in maintaining the most flow for the size unit.
This unit I bought is sold to car washes where they install them in series. Apparently, the idea has been in practice for some time.
I did the install myself. It does require a self-closing vent that was available from the same online vendor. Fortunately, my installation didn't require much repiping. I moved a few copper lines and ran several feet of black iron for the gas. The vent was very close to the old unit which used a hole in a window. I removed the window and installed the zero-clearance vent I had bought.
I have been doing a lot of work in the basement and I had installed the unit next to my washer dryer (I have basement laundries and I intend to remedy that soon). The burner had picked up some dust and other pollutants causing the burner to misfire.
I called tech support and they emailed me some docs on tearing the thing apart and cleaning it. It took me about two hours but was pretty simple even with the Japenglish instructions. I have to admit, I work on a lot stuff like this at my job in the merchant marine.
Anyhow, I have a four bedroom house, two young daughters and a wife who insists on running the washing machine way too much (in my opinion). We also have another adult living with us (I don't know why anymore). We have a dishwasher too. needless to say, the demand for water is pretty high. And we never run out.
As for the gas bill, I don't know anything other than the burner is incredibly efficient and we are burning only as much as we need (although I have been unsuccessful in convincing the others to conserve. They hate the needlely low flow nozzles).
The heater is designed to last decades. While I have no experience with deposits described in posts last year, I understand the stainless steel coils resist much of the deposits. However, the primary benefit in that regard compared to tanked systems is that tanks accumulate the sediment and get congested. The coiled tankless systems maintain constant flow through the coils and do not accumulate this type of sediement.
However, I might worry a little about the chemistry of raw water heated at boiling temperatures. This may cause some surface deposits. It has not been a factor as far as I can tell.
Did I mention that it comes with a remote control?