Pros: Endless hot water, Cheaper to run
Cons: Seems to take longer to get hot water flowing, Expensive
When we built our new home we opted for the Rinnai option that the builder offered. It was about a $2,000 option and we didn't install it ourselves, so that's why I left off the price since it can probably be had for less if you contract straight to the vendor.
The system has been great with no complaints. Our gas bill is about $8 in the summer when we are only using the Rinnai (no gas heat in use) and I think that's pretty good. We are a family of 4 running the washing machine at least twice a week as well as dishwasher 2 or 3 times a week. We bathe too.
If I had to find something to complain about (aside from the cost) it would be that it takes a solid 40 - 50 seconds to get hot water coming out of the tap (assuming it had not been in use for a while, like in the morning). I have been told this could be solved with a circulator pump, but that dissolves the efficiency of the Rinnai. I have also been told that it could just be the plumbing layout of our home. Regardless, it still seems a bit longer than is should take.
The system does seem to work as intended, providing endless hot water, there have been several instances of the washing machine running as well as a shower and a bath tub with no flux in hot water (water pressure is a different story, but no fault of the Rinnai).
Our unit is mounted in our garage next to our furnace. It's facing dimension is about that of a microwave door, and only sticks off the wall about 5 inches, so it is tiny, especially compared to a traditional hot water heater.
All in all, it's a great system, works as intended and if you can live with the cost as well as waiting a few seconds for hot water, this is for you.