I purchased this product for my 19 litre tropical aquarium (it was recommended as the tank is also a product of Interpet.)
So far, it has been brilliant. You can see when it's adjusting the water's temperature because of the red light switching on during heating, which is reassuring. I have a rather cold bedroom (where my fish tank is) and the heater seems to be on quite a lot, however, the fish (3 guppies and 2 danios) are comfortable and happy, and have been for a very long time so it must be doing it's job.
I can tell the water is warm from touching the glass and putting my finger in the water. I do not, however, have another thermometer to check the temperature so I cannot tell the accuracy of the heater, but as I mentioned, it's always seems right.
Setting the temperature is easy too, it's simply twisting the cap this way, or that way, depending on the temperature you want.
It's very durable and has stayed stuck to the glass, never had a problem and forget it's there. It's rather big, but I have a small fish tank so it probably just seems that way to me.
The power adapter is not too long, but it's not too short either, this can be easily fixed by an extension cord, of course, and the power adapter's WIRE can be submersed if need be safely.
Overall, I would definitely recommend this heater to anyone, it's from a reliable company and does it's job very well. I can't say whether it will be good in experienced or bigger/complex tanks but for startup/simple tanks, this is an exceptional product.