I love it
When our hot water heater went out I began gathering information on water heaters. Because I am on a well and live in a draught stricken area watching 4 gal. of water go down the kitchen drain every time I needed hot water was nerve racking. I was considering putting the new water heater on the back porch closer to the kitchen and redoing the plumbing. Then I saw this little gem. It is the only one I saw that you could install in line with your house hot water. My daughter had the same problem so I bought 2. It wash't bad to hook up. Our biggest problem was pulling out our dishwasher so my husband could change something and give me a plug in for it. We both love them. Turn the knob, hot water. The fact that it is in line with my big water heater I thought was a real plus and no one had to change around any plumbing or look at a water heater on the back porch.

Nice water heater
Overall, this product works very well and we love not having to wait for hot water at the kitchen sink. The form-factor is also very convenient for mounting under the kitchen sink, unlike some other small point-of-use water heaters.

I have this hooked up inline with a gas tankless water heater and I wanted to know how much this new heater was costing me in electricity, so I hooked up a meter to check it out for a few weeks. It turns out that, with our normal usage, it uses just about 5 KWH/week - which works out to about $2/month in our area.

Before this heater, we had to run a full 2 gallons of water down the drain before getting hot water to the sink - which was both a big pain and somewhat wasteful. With this 4 gallon heater, we basically have an instant and endless supply of hot water at the sink since the hot water from the main tankless unit arrives before the water cools too noticeably. I've also noticed that it seems to heat water on it's own very quickly.

The biggest problem with this water heater is how much of a pain it is to set the thermostat. You basically have to remove 3 panels and then turn a dial that isn't labeled at all. In order to get it set to the same temperature as my main water heater, I ended up using an instant-read thermometer and moving the thermostat dial as I filled the tank with cold water and let it reheat it to the set temp. The process took a while, but I shouldn't have to do that again.

Overall, I'm very pleased with the purchase and would recommend this water heater.

Hot Product
The heater does a great job of providing hot water immediately at the kitchen sink which is at the opposite end of the house from the water heater---no more wasting 2-3 gallons waiting for hot water. Installation was easy using flexible connections from the local hardware store, letting the heater simply sit on the floor under the sink. The heater even came with the pressure relief valve mounted. We are very happy with the performance of the heater.

Warranty issues
Please be aware that the manufacturer of Ariston brand does not have presense in US. I had purchased a washer/dryer from them in 2006 and it was sold to me with a full 2-year warranty (one of the reasons to choose it then). Now it is broken and they don't want to honor the warranty telling that they have no legal obligations to do so.

We love this water heater!
A lot of the reviews cite this product for use in addition to their water heaters for added hot water. However, if you are flat broke, without money to buy propane/gas or the electricity to run an electric water heater, then this is for you! We went three years with no hot water, having to heat it in canners on top of the stove and then carry it to the destination. With this heater we have plenty of hot water to wash dishes, wash your hands, whatever you need! When it runs out of water it takes about 30 min to heat again, but when we're just using it like you normally would throughout the day, we never run out of hot water. Now, in the winter when someone wants to take a bath, we heat one 4 gallon canner of water and then run the 4 gallons from the heater and get a piping hot bath! Since getting this for Christmas, we have not noticed any change in our electric bill. In the summer I think that the heater alone provide enough water for a bath (doesn't need to be as hot). We are even considering getting another one of these to use in the winter.
Okay, so the temperature is hard to adjust, I did it once (turned it to high) and I don't see why I ever need to adjust it again? To save electricity, when we're not home or at night, it has a on/off switch in the red bubble on the front that I just flip off. So instead of adjusting the temp (like you would on a regular water heater) to save energy, just turn the thing off!
Well worth the investment! Good luck!