Quoted from Yelobird:
I went with the Rinnai as it was the highest rated in every category. Cost a bit more but its built great. Never understand the water sandwich delay complaint. ALL water heaters have to clear the room temp water from the pipes till hot water is at the fixture? With the old water heater you would wait maybe 30 seconds to get hot water flowing, the tankless is almost the same time frame. Plus its only heating water when we need it unlike the 50 gallon boiler pot we had before. I could always add the recirc crossover valve if things change but for now I have no issues with the standard setup. As all plumbing is inside the house shouldn't matter where you live.
I’ve experienced the sandwich when staying with family in a cold climate. It was jarring. Something to do with minimum flow rate and time to restart. I had adjusted the shower down for a moment and 30 seconds or so later got a shot of ice cold water in my shower for about 10 seconds. Woke me up real quick.
I don’t think its as much of a problem with newer units though.