In this months Wired In, there's an article about them, all about a Voltis Home unit (more like a thinly veiled 2 page advert made to look like a magazine article)
As far as I can tell (there were few actual technical details) these units work on the principle that everything these days is designed to work on 230V, but we still have a nominal 240V supply.
So by simply reducing the supply from 240 down to 230, everything consumes less power. Well that's certainly true for heaters, cookers and filament light bulbs, but i'm not so convinced your computer and tv with switch mode power supplies designed to operate on any voltage, would make two jots of power saving on the reduced voltage.
I assume it's a little more sophisticated than an autotransformer, with presumably some form of voltage stabilisation so the output remains at 230v regardless of fluctuations in the input voltage.
It's a little sad that deficiencies in the supply side (like the fact we never did REALLY harmonise our voltage) creates what is almost an artificial market for what must be small energy savings.
There's a claim of "up to 17% energy savings" which for a purely resistive load would be about right. Reducing the voltage from 240 to 230 is a 4.2% reduction in voltage, so that would represent a 17.6% reduction in power of a purely resistive load.
Of course these "savings" are not real are they? For instance, your kettle would consume 17% less power, but it would take longer to boil, so you won't save any KWH usage (you cannae change the laws of physics jim) Likewise an immersion heater will take longer to heat your tank of water. Your shower will be colder, and by the time you have turned up the heat, the flow will be pathetic, so you will rush out and change your 8KW shower for a 9KW model to restore a decent water flow. An electric panel heater will take longer to reach desired temperature before the thermostat turns of etc etc
In fact about the only thing I can think that really will give a true energy saving, is a filament light bulb. That is unless you decide the lighting is too dim, and change it for a larger one. Perhaps it will make light bulbs last longer?
About the only other useful thing I can think they might do is power factor correction. How much of a saving is there to be had in a typical house by PFC alone?
On balance to me, it all sounds like a gimmick. do others agree or not? If these things take off in a big way, will it become the next miss-selling scandal when real savings prove to be tiny? (remember I made that prediction first)
The article did make the point this is largely unknown technology so selling it to consumers would be difficult.
My advice to anyone fitting them, is be there purely to fit something the customer has chosen. If you start recommending them and telling the customer how much they will save, you could end up in the brown and smelly stuff. (just my opinion)