This was an easy install with the exception of the previous water heater piping was offset from the actual heater. In other words the Rinnai V53DeN would have been even easier to install if the previous heater had been standardized.
So far it has worked quite well and for the price is excellent.
A good friend of mine just purchased a larger model based upon my high regard for the one I have. So far, he is also quite happy with the results

This is a review of the V53DeN. This is my second Rinnai tankless. The first was for the main house, this second for a master bedroom addition. I've had the first for more than 5 years and it's been wonderful. We didn't need 5.3gpm for a shower and a sink, but I trust Rinnai so I went with them again for the second heater. So far the unit has been great. My complaints have to do with the installation. I could have done it myself, but I hired a professional to help to make sure everything was to code. Neither of us could figure out the electrical from the enclosed instructions. We chased the wires in the diagram enclosed in the heater itself and figured out how to wire it, but no one should have to go through that effort to connect 3 wires. Also, the fitting for the electrical conduit on the bottom of the unit is machine thread not pipe thread. There must be a pipe-to-machine thread adapter that was missing from my unit. I will call Rinnai to find out. Again, the electrical shouldn't be this hard. It wasn't with our first unit. Finally, the box says "Wifi Enabled" but there's nothing in the instructions about how to connect. I don't care because I'm not connecting anyway, but it's just another way Rinnai really flubbed the documentation.
Edit: I finally finished the electrical connections 2 months later, no thanks to Rinnai customer help, who eventually stopped responding to my emails. The connection at the unit looks like 3/4" compression to me. I've attached pics in case they help anyone. It's a tight fit and getting at the cleanouts for yearly descaling is going to be tough. I'd skip this model and look for a different from Rinnai.

NEEDED TO EXCHANGE FOR A LARGER MODEL AS THIS ONE IS SO EFFICIENT AND QUIET BUT THE BTU RATING WAS TOO LOW FOR THE EXTRA LARGE SPACE WE HAD PLANNED ON HEATING. Dr Larry Fadjo

Installed this unit after Austin Texas snowpocalips in Feb. The old rinnai burst its heat exchanger because no power to run frost protection. I recommend draining water next time power goes out. Gas heat means nothing without a electrical igniter. New unit works fine.

I think this is a great buy. It installed easily (but needs mains electricity which most tank water heaters don't need, so plan on that wiring), no leaks, very tidy ...everything I was hoping for, except I underestimated the lag time before getting hot water. It is mounted just beyond the bathroom for primary use, but, even with a short run, most people are done washing their hands and disappointed before the warm water shows up. Its not the fault of the unit, just something you wouldn't think of until after. I like the unit, so when I have time, I will transfer it to our main house where it will service the bathrooms in the back half. It's design will definitely keep up with showers. I will just need to find a smaller, faster heating unit for the casita, but almost impossible to get small in natural gas.