We put one in 3 years ago. We are on Natural Gas and Private Well.
Would I do it again... Heck Yeah... I love it.
A couple things to note. If you have a traditional (Electric, LP or NG) and it fails. Replace it with another traditional water heater. If you want to make the switch the time to do so isn't when you don't have hot water. It takes a lot of planning. Things like how hot do you want your water. Meaning what is the point of use that you needs water the hottest? Shower, laundry, kitchen or garage? You then need to look at what your cold water temp is. Preferably in the winter. That is a bigger deal for those of us up north than down south. Those in warmer climates can get by with a much smaller tankless system. Your current traditional water heater is positioned not based on point of use but on where they can physically fit that beast. Does it make sense to put a tankless unit there or would you be better off moving it closer to your point of use? Afterall, closer to your point of use means it will cut down on the time until you get hot water and that waste. That is unless you get into re-circulation pumps and ****. Do you need a bigger gas supply line? I looked at electric until I found out in sizing that it would take 3 x 60A circuits to supply power and we only have 150A service and still need to run the well and it would be nice to not have to shower in the dark. You also need to figure out how much flow you need and the delta (change in temp from cold to hot) to size the system based on needs. We did have to upgrade the supply line into the house from 1/2 copper to 1" black pipe. The meter was fine. We are a little over the rating but not everything runs at once.
I read a lot about these before pulling the trigger. I read a lot of complaints about can't get hot water in a shower and low volume. We did our install in conjunction with a remodel of our main bath, more on how that comes into play later. Where some of the efficiency comes from is why heat water to 150F when you are just then cooling it back off to 105F by mixing in cold water to take a shower? You set the water heater at the temp that you want. Ours is set to 122F. If I need to I can ramp it up with a remote display. It is rare. Some people with the shower complaints are either used to setting the mixing valve at a certain position when they used to have 150F water coming in and find it too cold. Well yeah, you had to have the water heated to 150 to give you more capacity. You don't have capacity issues anymore. If things are set right you should be running 100% hot water and mixing in little to no cold. There is the first problem. Many integrated mixing valves (because of safety) always mix in some cold. That is why we are set to 122F. I like 110F showers from time to time. The other issue is if your hot water supply line to a large volume use point is big enough. Is that hot water line to the shower only 1/2". That was fine when you were running a 50/50 mix of hot/cold water. Now you might find your flow too low. As I mentioned we did the water heater in conjunction with a bathroom remodel. The shower was gutted as part of this and I have a 1" hot water line going to the shower. I still have to mix in a little cold because of the design of my mixing valve but I have no issues with running the shower head, cloud head and body sprayers all at the same time at 110F.
I will say they are not for everyone. We love it. With 2 kids one a teenage daughter, we never have to time showers anymore to make sure we are not going to run out. Have never had to rush through a shower. Any savings gained from more efficiency are probably eaten up by longer showers. What will the pay off be? In terms of $$ I don't care. In terms of happiness with the shower, the first time the entire family is getting ready for going out and everyone needs to take showers.
