I bought a Nordic Eco showerhead from The Cultivate Centre in Dublin on a money-back basis. I got that promise beforehand because at 50 euro, I didn't really fancy trying it and being stuck with something that doesn't work. I'm glad I did. This gadget may well be great but it is completely pointless for my shower.
In the brochure that comes with it, it says it will cut your shower from 22 litres/min down to 9 litres/min but it will still feel like a great shower. Sounds great. What I didn't realise is that my shower does only 4.5 litres/min! With the new shower head attached, nothing much changed and in fact the old shower head was better for that rate of flow.
The moral of the story is, if you're thinking about one of these, get a bucket or a jug or whatever and figure out how many litres/min your shower does before buying anything.
In hindsight I could have worked this out in advance, even without a bucket. It's a 10.5KWh shower. It takes 4000J to heat 1 litre of water by 1 deg C. I need to heat my water to about 40C and let's pretend it's starting at 0C (it's close enough in the winter). So it takes 40C*4000J/C = 160000J to heat 1 litre of my cold water to 40C. A shower that does that once per minute is using 160000J/60s = 2666W. So a 10500W can do that about 4 times per minute, i.e. about 4L/min.