Some things to consider:
1) only get the kind that run on natural gas. Electrical ones just dont do the job.
2) they really need to be within 30’ of where they are going to be used. Otherwise it can take a while before the hot water shows up
3) There really is no such thing as a whole house tankless despite what the manufactures say
4) Think of a smaller one to support at most 2 showers. Plus one for the kitchen (diswasher) and one for the the laundry. 3 for the typical home. Luxury homes may require 4 units
5) If your incoming water is really cold, you may even want a larger “whole house” to pre-heat the water. Most tankless will raise the water temp at least 30 degrees at full flow. So the series approach will raise the incomming from about 55 degrees to 85 degrees. The final will then raise the 85 up to something close to or over 120 degrees which is approaching a burn if left too long.
Now, if you build them that way, you will have lots of never ending REALLY hot water that may be less expensive than a conventional. The trade off is obviously more equipment, installation costs (running the gas lines and vents), and loss of space.
Is it WORTH it? Depends.
If the question is more of a technical, will it pay for itself in cost savings over it’s life time. Possibly.
If you factor in the additional convenience ... probably.
Does it compare to other systems? That gets harder to say. I know one guy who pulled out his gas / force air furnace and went to a wood pellet boiler. He has his boiler piped so that some of the boiler water heats his hot water tank. His hot water in the winter time is essentially free. Now understand he lives up on a mountain in Colorado where nightly winter temps are often below 0. He also gets pellets at around $210 per ton.
I know another guy who installed two additional 40 gallon tanks (total of 120 gallons), added an water heater blanket to each, then did a solar assist to preheat the water coming in. That to was a lot of installation and work, but most of which he could do himself. He lived out on the eastern plains of Colorado and got lots of sun daily.
Your mileage may (likely) will vary
While all this thread has been happening, I've been researching and making some calls and ...
I use LPG for cooking and I have a 100 gal tank that lasts me at least 3 years ... but that's only doing the cooking.
Does ANYone know about gas usage with an LPG tankless?
I DO understand differences in home sizes and people, etc.
We are two empty nesters in a mobile home