To test the effectiveness of the IntelliPlug system, I plugged my work computer and two monitors into the standard version and measured the power drawn with the computer on and off. I then plugged the computer and monitors into a standard extension cable and did the same test, once with the monitors left on standby and once with them turned off manually using the buttons on the front. When turning the monitors off manually, they drew tiny amounts of power and the difference between using the IntelliPlug and just a normal extension cable was pretty minimal and certainly not worth the £15 outlay for the plugs themselves. In fact, the computer was using 7W while idling, so it was far and away the most power hungry part. However, if the monitors are left on standby, the difference becomes all too apparent and the IntelliPlug starts to justify itself.
Now, you could argue that turning off your monitors isn't that much of an inconvenience, and you'd be quite right. However there are four counters to that argument. Primarily is the fact that, it genuinely is a bit of a pain to go round turning off all your equipment at the end of the day, especially if you want certain things left on and not others, so you can't just turn the lot off at the wall. Secondly, I only use two 17inch monitors but if you have two 30in displays, or use the plug for something more power hungry, even when the device is 'off', you will see a larger difference. Third, using the plug to power equipment that is otherwise inconvenient to turn off fully - like routers and modems for your computer, or set top boxes and DVD players for your TV - makes a total power down a lot easier. Finally, while there are only two sockets on the IntelliPlug, you can plug standard extension cables into these and power many more devices and, when you combine the very small trickle of power from many devices, the power savings begin to add up.
Ultimately, the IntelliPlugs and IntelliPanels are primarily labour saving devices more than power saving angels. If you can easily reach all the mains switches, to turn your equipment off, then you'll save more power doing that. However, if like me you favour the idea of just pressing the power button on your TV remote and turning off your DVD player, surround sound system, HTPC (and associated external water cooling kit), and gaming systems all at once, you'll find these little plugs indispensable.
The OneClick IntelliPlug is one of those ingeniously simple devices that once used, you'll wonder how you ever lived without it. It may not be the most effective way to save power, and is really more of a labour saving device than anything else, but if you have a lot of AV and IT devices around your home, I can't recommend it highly enough.