Rinnai installed in 2012, not problem one.. 
. Two showers,dishwasher and washing machine all running, never out of hot water



Amen. I love the hot showers. I have my temperature between "scald a hog," and "boil a lobster," and it never runs out.
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But not needed for the more common gas units?
It might make more sense for new construction. Several years ago when I was considering getting one, I remember it was going to cost me over $4K to get one installed. At the time I had lost a hot water tank heater (after about 20 years of use) and thought tankless would be neat to have. I installed a new tank heater myself, cost me less than $500 for the tank from Home Depot. $500 vs $4000? Hmm....... tough decision.