i have a Rinnai and it has been great for about 10 years. I recently flushed it for the first time and didn't get nearly as much crude out of out as I thought. I didn't install it but I don't think it would be too difficult.
Make sure to check the gas piping to insure enough gas flow, and keep in mind if you have something downstream (like your furnace or a gas dryer). my furnace is downstream so I ended up having a larger pipe installed to the point of the water heater. There are charts online that show what size pipe you need for the distance and the BTU that the appliance uses. Those things need a healthy amount of gas.