PowerSwitch

Theres a new fangled device on telly at the moment, V Phase.
It saves you 10% on your electricity bill, by lowering your voltage to 220volts.
Unless you draw over 10 amps, when it shuts down.

Obviously it doesnt work, because heating elements will just run longer, motors will draw more amps and light bulbs will just be replaced for brighter ones.

Yes, they are a con trick, and may actually increase the electricity bill due to the loses in the device.
Different types of electrical load react differently to a reduction in voltage, the position may be summarised as follows.

Incandescent lamps including halogen types will use slightly less power but give appreciably less light. Cost per lumen increased.
If the dimmer light is acceptable, then it would have been better to select fewer or lower powered lamps.
If the dimmer light prompts the user to select higher powered lamps then the bill will increase.

Flourescent and discharge lamps on copper/iron ballasts will use less power but give less light, roughly in proportion to the reduced consumption. Cost per lumen about the same.
If the reduced light level is acceptable, then it could have been achieved by fewer or lower powered lamps.

Flourescent and discharge lamps on electronic ballasts will generaly give the same light and use the same power, no saving.

Switched mode power supplies will use the same power at the lower voltage, no saving.

Thermostaticly controled water or space heating will use less power whilst energised, but will have to run for longer to achieve the desired result.
Total cost to heat a room or a water tank is unchanged.

Non thermostatic heaters will use less power, but produce less heat in exact proportion. The same effect would be obtained by useing a smaller heater. Electric showers will produce either a smaller flow of heated water, or water heated to a lower temperature.

Induction motors will draw more current, run hotter, and be at increased risk of failure.
Comutator motors will run at reduced speed and therefore probably have to run for longer to do the same work.

There are many voltage reducing devices on the market, all use transformers, the consumer is therefore paying forever for the iron losses in the transformer.
These devices are popular in the public sector, but that does not prove that they work. In extensive premises, the voltage at far points may become too low for correct operation of some loads.

Some users report significant reductions in bills, but only when voltage reduction is installed at the same time as genuine energy saving measures, from which of course the saving derives

Avoid

I think that maybe a little unfair.

The answer is it depends, DNO's (distribution network operators) are meant to supply the electrical network with a nominal 230V.
They don't, if you are close to a substation your supply will be higher (250V plus is possible).
This can have a detrimental effect on motors, which will expire quicker and you will have higher electricity bills.
As the V-phase units switch off when under a 10A load or more, (they are working on a version that will take a higher load)
the manufacture recommends having circuits that exceed 10 A, separate ie. cookers, immersion heaters and showers.

Commercial voltage reducers have been used for some and are very cost effective, domestic versions less so at the moment.
However if you need to change your consumer unit, have high voltages and wondered why you go through so many hair driers and toasters.
A V-Phase maybe for you.

I can not agree that these devices are useful in any common circumstances.
They are certainly used in some commericial premises, but to me that shows what a good sales team the supplier has, not that the product is any good.
The nominal or declared voltage is 230 volts, and the permited range is from 217 volts up to 253 volts.
All domestic appliances should work correctly from 210 volts up to 253 volts. In practice though performance tends to suffer at the lower end of the range.
The actual voltage supplied most of the time in most places tends to be nearer 240 than 230.

Many appliances will use less power at the lower voltage, but will also do less work, a toaster for example will take longer to toast, and a kettle take longer to boil.
The cost per slice of toast or litre of boiled water would be very slightly higher at the lower voltage, the difference is minute and of little importance, but is NOT a saving.

Incandescent lamps will certainly last a lot longer on the lower voltage, but the already poor efficiency of incandescent lighting will be made even worse.
The saving on lamps appeals to some people, many hate buying lamps but dont worry about electricity.

A lightly loaded motor might be more efficient at the lower voltage, but a fully loaded one will be likely to use more current and run hotter.

In large premises the increased current at the lower voltage can overload wiring and trip MCBs. Losses in wiring will also increase, but should still be a very small part of the load.

I think it's also true that too-low voltages can cause the inverters on any renewables they may haveinstalled, to trip out as if there were a proper power cut.

i did a bit of a google before posting, the opinion on electricians forums was that there are some industrial applications for units that cost 30k+.
None for domestic units