Our house had a Noritz model when we moved in. I can't tell you about the cost savings since I have nothing to compare to, but I can make some general comments.
1. It definitely keeps up with demand for our usage. We regularly have 2 people taking hot showers at the same time.
2. Ours uses gas to heat the water but needs electricity to run. Get some sort of backup electrical system if you want hot water during power outages. (That's my next project!)
3. On our model, water pressure plays into the ability to ignite the gas to start heating. How do I know? When we first moved in I wanted to test the water heater out. So I turned on the washer, hot water in the sink, the dishwasher, and a hot shower all at once to see if it could take it. The pressure drop killed the water heater. I turned up the water pressure in the house a bit and tried again and all was fine. I did need to reset the water heater by flipping the fuse at the fusebox. Every once in a while we'll need to reset the water heater at the fusebox if we get some sort of sudden pressure drop during use.
4. The only problem I have is that it takes a long time (> 1 minute) for hot water to get to the kitchen sink during the winter. The kitchen is the farthest away from the water heater. I don't consider this too big of a deal for our use, but YMMV.
