I plumbed our Navien 210A in 3/4" copper pipe this summer and it works beautifully! I skipped the flexible stainless hoses and plumbed directly to the tankless shutoff valves in 100% soldered copper. I spent many hours with the blowtorch, so the installation is solid! I hired a master plumber for $300 to do the gas hook-up. It took him almost four hours. I couldn't be happier with how everything works.
Because of our very cold groundwater in Wisconsin, I installed the 210A downstream of our 25 year old 50 gallon AO Smith gas water heater which provides baseline pre-warming to 60 degrees. We never experienced any hot water shortage with this arrangement. Without the pre-heater, Navien's larger 240A would probably have been a better choice for our climate. Eventually I'll have to replace the AO Smith tank, but it's showing no sign of leaks or other problems.
I understand de-liming is a routine part of tankless maintenance. All water heaters have some maintenance. Having installed a pair of 5 inch three-stage water filters upstream of the tankless inflow dramatically reduces the amount of debris entering the heater, and that hopefully improves our efficiency between cleanings. I haven't needed to change filters yet. I expect to get about 8-12 months between filter changes.
I used 100% PVC venting. The discussion about high pipe temps is nonsense. The exhaust pipe hardly rises a couple degrees above basement room temp, which is around 60 degrees in the winter and 70 degrees in the summer. There is NO possibility of overheating the PVC vent. It's not even minimally warm to the touch.
Our total gas bill for our family of four in October was $19 (including central HVAC heat.) We have a 2600 square foot home plus full basement. November's gas bill was $30 -- thanks in part to supplemental home heating with a wood stove. I estimate our Navien gas usage at around $10-15 per month. We'll have exact numbers next summer when the HVAC is off, since we don't have any other gas appliances besides the water heaters and HVAC. We average around 18 showers per week (2 adults, 2 kids) plus lots of laundry in our 20 year old Maytag washer, our dishwasher, and plenty of hot running taps. We live comfortably with no sweaters in the winter, no scrimping on showers or laundry. My wife likes a scalding 20 minute ~115 degree shower. Mine and the kids are 12-15 minutes at (I'm guessing) 105-110 degrees. Our Navien is set at the default of 120 degrees, so our delta temp is about 60 degrees above prewarming.
I meant to post some photos of our installation here but didn't get around to doing it yet. Will try to make time for that soon.
Again, we have no regrets about our Navien tankless water heater. It's been everything we hoped -- reliable, quiet, efficient and very smart. My only surprise is that we don't need the recirculator tank to avoid hot-cold stacking, so I turned it off. I guess there's no real downside to having the recirculator except more complexity and potential for future failure. I would probably buy the non-recirculating Navien model if I had it to do over, but it's a moot point now.