At the time, back in 2008, the average price of an electric hot water was $400 or so. I got one of the Rinnai propane tankless heaters and it was ~$1500. A typical electric hot water heater would cost between $400 and $500 per year to operate. Compared to the electric heater, I calculated that I'd make up the extra 1k in 7-7.5 years or so. The electricity savings have been much greater than I expected. For example, my power bills run roughly $90-$100 a month, year round. My neighbors have houses 1/2-1/3 smaller than mine, and pay twice as much each month. Not all of my savings are attributed to the water heater, but a decent chunk of it is.
I've never had any issues with having enough hot water. That has more to do with pressure than anything else. As long as you've got the pressure pushing the water to your fixture, it'll be hot. I've taken a shower while the dishwasher and washing machine were both going and there was no difference except for a minimal loss in pressure.