I have one.
I have one and I love it. Or I should say the wife loves it. She can spend an hour in the shower and never run out of hot water. Ever want to was the truck with hot water? This is your unit.
I have had mine for 3 years and it has only given me trouble once. The main shutoff for the gas supply stuck. After some lube, it works fine. No other problems whatsoever. It will add up to 90 degrees to your incoming water. Ours is around 45 degrees right now, so we get hot water to the tune of 120-130. Plenty hot! Easy to install (relatively speaking). There is also a 20 year warranty on the tube bundle. I would highly recommend a water softener though, if you don't already have one. You can see the one I have at
www.cechot.com These are the ones you see at Menards (where I got mine) and Home Depot.
The only downside to it is that you can't use more than one hot water faucet at a time. You can but it is really slow. So don't try to take a shower while the old lady is doing laundry. If you get the bigger (3/4" unit), you may be able to get away with it. I have the regular (1/2" unit), and it sometimes gets tight on supply. But if you have a large family, you can't go wrong.
My energy bills definately reflected the savings. Although, my previous heater was electric. I think it has paid for itself by now. I've saved over $20 a month. I don't think you'll be disappointed.
If you have any more questions, let us know.