Very good
Installation: easy
The instructions are not great, but you just need to connect a power supply and two water sources.
Note 1
Attention! The device is 4.4kw. This means that you need to use a cable like the one for the water heater and connect it to a 16A fuse line.
Note 2
The package includes 2 filters.
One fine and one coarse. The instructions show how to connect them, but while the fine one fits the 1/2" cold water pipe, the coarse one doesn't fit anywhere. No matter how much I searched, I couldn't find a place for it. Maybe the coarse one is meant for a 3/4" water supply.

Tip: Install a power switch on the supply line so that in case of a water leak, you can turn it off until the repair is done, to avoid wasting electricity.

Performance: very satisfactory. In pure hot mode, the water is scalding. The temperature cannot be adjusted. It is fixed. You just mix it with cold water at the faucet.
Water heating speed: as soon as you open the hot water at the faucet, you just wait for the distance between the outlet connection and the faucet to be covered, which takes about 3-4 seconds...
Flow: due to the installation of the filter (which is necessary to prevent foreign objects from entering the circuit and causing resistance), the flow of pure hot water is very low. It does scald, but it runs very slowly. However, if you wash with a mix of cold water, the pressure and flow become normal.
Quality: it seems durable. Now, how long it will last... we shall see!!
Personally, I am completely satisfied with the result. I don't consider it a waste of money.