I installed a Rinnai tankless hot water system last year fueled with propane. I converted from a 60 gallon electric system. The savings are tremendous. It will pay for itself within 2-3 years. The cost of the system with professional installation is high compared to a normal hot water tank. But with our mineral laden water I was only getting 7-8 years out of these tanks. Since the water flows only when requested there is no sediment to rot the coils or the tank. The upsides are, less energy cost, no tanks to replace, and unlimited hot water. Unlimited hot water is nice when you have 3 to infinity # of people who want to all take a shower.
The only downside I see is that instead of waiting for 8-10 seconds for the hot water to flow through the pipes from a tank it takes 15-20 seconds from the Rinnai.