tried them and ended up having to take one out from one customers as I discovered the flaws with them - though we've still got one running at our house.
Basically all the whole house units are fixed voltage reduction units, so for example the Apex unit is fixed at 18V reduction, and it then switches to bypass if the voltage output drops below something like 214V output / 232V input.
Unluckily, in this situation it turned out that northern powergrid had unilaterally decided to turn the entire regional grid down by 6V between our survey and the installation of the VO unit, and it was at the end of a long line, so while it was up around 250V at peak, around 5-8pm it was hovering around 232V so the unit spent the entire period rapidly cutting in and out, and causing regular 18V swings in the household voltage.
I checked every other unit on the market out after this, and while some of them do give you the option to have different set voltage reduction points eg 12V or even as low as 5V - really what's the point if it's only going to be that low a reduction.
tbf to the Vphase unit I think that is supposed to actually stabilise the voltage around 220V, however it only does 8amps continuous, 20 amps peak, so can only be used for a small proportion of the circuits.
Actually, we have got one unit in at a place with a swimming pool, and generally very high bills, and I think that's working ok, though we've done a load of work there so it's hard to tell what impact it's having.
So my verdict would be that for domestic they're probably only worthwhile in a relatively low proportion of cases with particularly high energy users and relatively stable high voltages. Northern Powergrid reducing the grid voltage by 6V probably reduces the need for them a lot as well, and makes it seems a bit unwise to be putting them in elsewhere as they might follow suit, which could then lead to problems down the line.
we have a unit sitting in our warehouse if you want to take a functioning but slightly used unit for a test, I'd be open to an offer on it.